TypeError when writing to excel using numpy.timedelta64
Let me start by saying that I apologize if this is already on your radar.  I looked, but could not find a similar issue.  When I try to write out numpy.timedelta64 to excel I get the following error:

```
pandas.__version__
Out[22]: '0.11.0.dev-6cda5dc'

run tester
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/Users/bnice/.virtualenvs/ipython/lib/python2.7/site-packages/IPython/utils/py3compat.pyc in execfile(fname, *where)
    176             else:
    177                 filename = fname
--> 178             __builtin__.execfile(filename, *where)

/Users/bnice/Source/latchmatchcatch/bin/tester.py in <module>()
     27 outputter = writeout_string + '.xlsx'
     28 writer = pandas.io.parsers.ExcelWriter(outputter)
---> 29 df.to_excel(writer, 'results', index=False)
     30 
     31 

/Users/bnice/.virtualenvs/ipython/src/pandas/pandas/core/frame.pyc in to_excel(self, excel_writer, sheet_name, na_rep, float_format, cols, header, index, index_label, startrow, startcol)
   1407         formatted_cells = formatter.get_formatted_cells()
   1408         excel_writer.write_cells(formatted_cells, sheet_name,
-> 1409                                  startrow=startrow, startcol=startcol)
   1410         if need_save:
   1411             excel_writer.save()

/Users/bnice/.virtualenvs/ipython/src/pandas/pandas/io/parsers.pyc in write_cells(self, cells, sheet_name, startrow, startcol)
   2192                             'cur_sheet property')
   2193         if self.use_xlsx:
-> 2194             self._writecells_xlsx(cells, sheet_name, startrow, startcol)
   2195         else:
   2196             self._writecells_xls(cells, sheet_name, startrow, startcol)

/Users/bnice/.virtualenvs/ipython/src/pandas/pandas/io/parsers.pyc in _writecells_xlsx(self, cells, sheet_name, startrow, startcol)
   2210             colletter = get_column_letter(startcol + cell.col + 1)
   2211             xcell = wks.cell("%s%s" % (colletter, startrow + cell.row + 1))
-> 2212             xcell.value = _conv_value(cell.val)
   2213             if cell.style:
   2214                 style = CellStyleConverter.to_xlsx(cell.style)

/Users/bnice/.virtualenvs/ipython/lib/python2.7/site-packages/openpyxl/cell.pyc in _set_value(self, value)
    322     def _set_value(self, value):
    323         """Set the value and infer type and display options."""
--> 324         self.bind_value(value)
    325 
    326     value = property(_get_value, _set_value,

/Users/bnice/.virtualenvs/ipython/lib/python2.7/site-packages/openpyxl/cell.pyc in bind_value(self, value)
    311                 self.set_value_explicit(value, self.TYPE_NUMERIC)
    312                 return True
--> 313         self.set_value_explicit(value, self._data_type)
    314 
    315     def _get_value(self):

/Users/bnice/.virtualenvs/ipython/lib/python2.7/site-packages/openpyxl/cell.pyc in set_value_explicit(self, value, data_type)
    228             self.TYPE_BOOL: bool, }
    229         try:
--> 230             self._value = type_coercion_map[data_type](value)
    231         except KeyError:
    232             if data_type not in self.VALID_TYPES:

/Users/bnice/.virtualenvs/ipython/lib/python2.7/site-packages/openpyxl/cell.pyc in check_string(self, value)
    200         # convert to unicode string
    201         if not isinstance(value, unicode):
--> 202             value = unicode(value, self.encoding)
    203         value = unicode(value)
    204         # string must never be longer than 32,767 characters

TypeError: coercing to Unicode: need string or buffer, numpy.timedelta64 found

```

My code to generate this error is as follows:

```
import pandas, numpy, StringIO, datetime

txt = '''ID,DATE
002691c9cec109e64558848f1358ac16,2003-08-13 00:00:00
002691c9cec109e64558848f1358ac16,2003-08-13 00:00:00
0088f218a1f00e0fe1b94919dc68ec33,2006-05-07 00:00:00
0088f218a1f00e0fe1b94919dc68ec33,2006-06-03 00:00:00
00d34668025906d55ae2e529615f530a,2006-03-09 00:00:00
00d34668025906d55ae2e529615f530a,2006-03-09 00:00:00
0101d3286dfbd58642a7527ecbddb92e,2007-10-13 00:00:00
0101d3286dfbd58642a7527ecbddb92e,2007-10-27 00:00:00
0103bd73af66e5a44f7867c0bb2203cc,2001-02-01 00:00:00
0103bd73af66e5a44f7867c0bb2203cc,2008-01-20 00:00:00
'''
df = pandas.read_csv(StringIO.StringIO(txt))
df = df.sort('DATE')
df.DATE = pandas.to_datetime(df.DATE)
grouped = df.groupby('ID')
df['X_SEQUENCE_GAP'] = pandas.concat([g['DATE'].sub(g['DATE'].shift(), fill_value=0) for title,g in grouped])

now = datetime.datetime.now()
timestamp = now.strftime('%Y%m%d%H%M%S%f')
outfolder = 'output'
out_file_name = 'results'
writeout_string = '../{0}/{1}-{2}'.format(outfolder, out_file_name, timestamp)
outputter = writeout_string + '.xlsx'
writer = pandas.io.parsers.ExcelWriter(outputter)
df.to_excel(writer, 'results', index=False)
```

There is an easy workaround.  I can convert the delta to a string in my code.  I just wondered if this was expected behavior?
