Concatenation can fail when indexes use nanoseconds and tseries.index.union_many is called
Concatenation can fail when indexes use nanoseconds and tseries.index.union_many is called. 

```
def thisFails():
            # May be related to  https://github.com/pydata/pandas/issues/1238 
            timeStep     = np.timedelta64( 1, 'ns' )
            t0     = np.datetime64( datetime.datetime.now() )
            ndx1   = [t0 + k*timeStep for k in xrange( 5 ) ]
            ndx2   = [t0 + (k-1)*timeStep for k in xrange( 5 )]
            df1    = pd.DataFrame( data = list( xrange( 5 ) ), index = ndx1, columns = ['Apples'] )
            df2    = pd.DataFrame( data = list( xrange( 5 ) ), index = ndx2, columns = ['Bananas'] )
            _both   = pd.concat( [df1, df2], axis = 1 )
            print "Failed to replicate bug"

        def thisWorks():
            # May be related to  https://github.com/pydata/pandas/issues/1238 
            timeStep     = np.timedelta64( 1, 'ms' )
            t0     = np.datetime64( datetime.datetime.now() )
            ndx1   = [t0 + k*timeStep for k in xrange( 5 ) ]
            ndx2   = [t0 + (k-1)*timeStep for k in xrange( 5 )]
            df1    = pd.DataFrame( data = list( xrange( 5 ) ), index = ndx1, columns = ['Apples'] )
            df2    = pd.DataFrame( data = list( xrange( 5 ) ), index = ndx2, columns = ['Bananas'] )
            _both   = pd.concat( [df1, df2], axis = 1 )
            print "Failed to replicate bug"

        try:
            thisFails()
            raise ValueError(" This was expected to fail. It might be that pandas has been updated from 0.11.0 ")
        except:
            pass

        thisWorks()
```

The error message is 

```
  File "D:\ATHENA~1\RELEAS~1\build\ext2\vc90_xp/lib\python2.6\site-packages\pandas-0.11.0-py2.6-win32.egg\pandas\tools\merge.py", li\
ne 872, in concat

  File "D:\ATHENA~1\RELEAS~1\build\ext2\vc90_xp/lib\python2.6\site-packages\pandas-0.11.0-py2.6-win32.egg\pandas\tools\merge.py", li\
ne 942, in __init__

  File "D:\ATHENA~1\RELEAS~1\build\ext2\vc90_xp/lib\python2.6\site-packages\pandas-0.11.0-py2.6-win32.egg\pandas\tools\merge.py", li\
ne 1104, in _get_new_axes

  File "D:\ATHENA~1\RELEAS~1\build\ext2\vc90_xp/lib\python2.6\site-packages\pandas-0.11.0-py2.6-win32.egg\pandas\tools\merge.py", li\
ne 1130, in _get_comb_axis

  File "D:\ATHENA~1\RELEAS~1\build\ext2\vc90_xp/lib\python2.6\site-packages\pandas-0.11.0-py2.6-win32.egg\pandas\core\index.py", lin\
e 2705, in _get_combined_index

  File "D:\ATHENA~1\RELEAS~1\build\ext2\vc90_xp/lib\python2.6\site-packages\pandas-0.11.0-py2.6-win32.egg\pandas\core\index.py", lin\
e 2728, in _union_indexes

  File "D:\ATHENA~1\RELEAS~1\build\ext2\vc90_xp/lib\python2.6\site-packages\pandas-0.11.0-py2.6-win32.egg\pandas\tseries\index.py", \
line 874, in union_many

  File "D:\ATHENA~1\RELEAS~1\build\ext2\vc90_xp/lib\python2.6\site-packages\pandas-0.11.0-py2.6-win32.egg\pandas\tseries\frequencies\
.py", line 431, in to_offset
ValueError: Could not evaluate N
Locals:
          _: ''
      delta: 'None'
    freqstr: 'N'
     stride: ''
stride_sign: 'None'
```

Apologies if this has already been fixed. 
