DOC: Clarified documentation for MultiIndexes

Add examples and rewrite an awkwardly-written bit

Cross-reference MultiIndex constructors
Merge pull request #5964 from felixlawrence/improve-multiindex-docs

DOC: Clarified documentation for MultiIndexes
DOC: release.rst mention IPython warnings in qtconsole fix and workaround GH5922
CLN: remove util.counter, duplicates compat
BLD: travis, Install apt-get deps first
BLD: travis, future-proof pip args
Merge pull request #6017 from y-p/PR_cln_counter

Remove duplicated code and fix travis
ENH: improved inference of timedelta64[ns] and datetime64[ns] in the prescence NaT/None/NaN
     from object dtypes (GH5689)
CLN: refactor of timedelta routines to cython for inclusion in maybe_convert_objects
BUG: apply to a Series with a timedelta (GH5458)
ENH: improved timedelta inference for non-ns dtypes
BUG: timedelta64 merge/join issues (GH5695)
DOC: release note for timedelta changes

TST: skip tests on numpy < 1.7 as needed for timedeltas

BUG: fixup .abs for timedeltas