API: stub-implementation of Float64Index (meaning its the 'same' as an Index at this point), related GH263

BUG: make sure that mixed-integer-float is auto-converted to Float64Index
BUG: (GH4860), raise KeyError on indexing with a float (and not a floating index)
CLN/ENH: add _convert_scalar_indexer in core/index.py
CLN/ENH: add _convert_slice_indexer in core/index.py

TST: slice indexer test changes in series/frame

BUG: fix partial multi-indexing