UnicodeEncodeError from DataFrame.to_records
The `DataFrame.to_records` method fails with a `UnicodeEncodeError` for some unicode column names.

(This issue is related to https://github.com/pydata/pandas/issues/680.  The example below extends the example given in that issue.)

```
In [322]: df = pandas.DataFrame({u'c/\u03c3':[1,2,3]})

In [323]: df
Out[323]: 
   c/σ
0    1
1    2
2    3

In [324]: df.to_records()
---------------------------------------------------------------------------
UnicodeEncodeError                        Traceback (most recent call last)
<ipython-input-324-6d3142e97d2d> in <module>()
----> 1 df.to_records()

/redacted/python2.7/site-packages/pandas/core/frame.pyc in to_records(self, index, convert_datetime64)
   1013             elif index_names[0] is None:
   1014                 index_names = ['index']
-> 1015             names = index_names + lmap(str, self.columns)
   1016         else:
   1017             arrays = [self[c].get_values() for c in self.columns]

UnicodeEncodeError: 'ascii' codec can't encode character u'\u03c3' in position 2: ordinal not in range(128)
```
