cut with np.inf gives TypeError
``` python
import pandas as pd
import numpy as np
arr = np.random.randn(20)
factor = pd.cut(arr, [-np.inf, 0, np.inf])
```

gives this error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/weitzenfeld/anaconda/lib/python2.7/site-packages/pandas/tools/tile.py", line 102, in cut
    include_lowest=include_lowest)
  File "/Users/weitzenfeld/anaconda/lib/python2.7/site-packages/pandas/tools/tile.py", line 168, in _bins_to_cuts
    include_lowest=include_lowest)
  File "/Users/weitzenfeld/anaconda/lib/python2.7/site-packages/pandas/tools/tile.py", line 204, in _format_levels
    fa, fb = fmt(a), fmt(b)
  File "/Users/weitzenfeld/anaconda/lib/python2.7/site-packages/pandas/tools/tile.py", line 200, in <lambda>
    fmt = lambda v: _format_label(v, precision=prec)
  File "/Users/weitzenfeld/anaconda/lib/python2.7/site-packages/pandas/tools/tile.py", line 247, in _format_label
    return sgn + '%d' % whole
TypeError: %d format: a number is required, not numpy.float64

I'm using pandas version 0.12.0.  Removing the -np.inf and np.inf eliminates the error.  
