select_dtypes fails on float16 typecode
I'll see if it's an easy fix.

```
import pandas as pd
import numpy as np

FLOAT_TYPES = list(np.typecodes['AllFloat'])

df = pd.util.testing.makeDataFrame()
df.select_dtypes(FLOAT_TYPES)

FLOAT_TYPES.remove('e')
df.select_dtypes(FLOAT_TYPES)
```
