Merge pull request #4855 from jreback/axis_len

CLN: _axis_len checking cleanup and better message
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: (GH4860), raise KeyError on indexing with a float (and not a floating index)
API: (GH263). remove fallback indexing on Float64Indexes for consistency