ENH: Add 'is_' method to Index for identity checks

Adds a method that can quickly tell whether something is a view or not
(in the way that actually matters).

CLN: change 'is' check to 'is_' check in 'equals()'
Merge pull request #4909 from jtratner/add-is_-method-to-index

ENH: Add 'is_' method to Index for identity checks
ENH: Added xlsxwriter as an ExcelWriter option.

Added xlsxwriter as an optional writer engine. Issue #4542.
Merge pull request #4857 from jmcnamara/enh_xlsxwriter2

ENH: Added xlsxwriter as an ExcelWriter option.
Merge pull request #4942 from jreback/index_cache

BUG: Fixed a bug in DataFrame/Panel cache insertion and subsequent indexing GH4939
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