BUG/ENH: provide better .loc based semantics for float based indicies, continuing not to fallback (related to GH236)

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

DOC: release notes / v0.13.0
TST: tests for doc example

CLN: core/indexing.py clean
DOC: add to indexing.rst

DOC: small corections

API: make Float64Index getitem/ix/loc perform the same (as label based) for scalar indexing

TST: Float64Index construction & tests

TST: misc tests corrected in test_series/test_reshape/test_format

TST: Float64Index tests indexing via lists

DOC: additional example for float64index
Merge pull request #4850 from jreback/loc_float

BUG/ENH: provide better .loc based semantics for float based indicies, continuing not to fallback (related GH236)