Incorrectly assigned Index ops/names

[jbrockmendel]

Index op naming and pinning is inconsistent:

• no Index.__rsub__, https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/base.py#L2200
• __iadd__, __isub__ aren't getting pinned to cls https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/base.py#L3998
• __radd__, __rpow__, __rmul, ... aren't getting the correct names (not such a big deal)
• numeric subclass methods names:

>>> pd.Int64Index.__add__.__name__
'_evaluate_numeric_binop'
>>> pd.Float64Index.__mul__.__name__
'_evaluate_numeric_binop'
>>> pd.DatetimeIndex.__radd__.__name__
'__add__'

[gfyoung]

I might also add that _evaluate_numeric_binop is not a particularly useful name.