Infer Decimal dtype
```python
In [15]: from decimal import Decimal

In [16]: s = pd.Series([Decimal(3), Decimal(1), Decimal(4)])
    ...: s
Out[16]: 
0    3
1    1
2    4
dtype: object

In [17]: s.dtype
Out[17]: dtype('O')

In [18]: pd.lib.infer_dtype(s)
Out[18]: 'mixed'

In [19]: set(map(type, s))
Out[19]: {decimal.Decimal}
```
It would be nice if the `infer_dtype` function could determine if the column was all of decimal type which could be useful when working with databases which return data as decimal types