DEPR: Indexers will warn FutureWarning when used with a scalar indexer this is floating-point (GH4892)

closes #4892

numpy is doing this i 1.9 IIRC, and Scipy already did it. in 0.13.3

 Indexers will warn ``FutureWarning`` when used with a scalar indexer and a non-floating point Index

```
# non-floating point indexes can only be indexed by integers / labels
In [1]: Series(1,np.arange(5))[3.0]
        pandas/core/index.py:469: FutureWarning: scalar indexers should not be floating point unless a    floating index
Out[1]: 1

In [5]: Series(1,np.arange(5)).iloc[3.0]
        pandas/core/index.py:463: FutureWarning: scalar indexers should not be floating point unless a floating index
Out[5]: 1

# these are Float64Indexes, so integer or floating point is acceptable
In [3]: Series(1,np.arange(5.))[3]
Out[3]: 1

In [4]: Series(1,np.arange(5.))[3.0]
Out[4]: 1
```