BUG: Fix DatetimeIndex.insert() with non-datetimes.

Coerces to object Index instead.
COMPAT: allow string concats to datetimeindex only (int/float are invalid)
BUG: Fix to_datetime to properly deal with tz offsets #3944

Currently for certain formats of datetime strings, the tz offset will
just be ignored.
DOC: moved numpydoc sphinx extension to subdirectory

Clearer distinction from ipython directive files
DOC: update numpydoc to current master (commit 223df02530)
DOC: add guidelines for building the docs
DOC: add section contributing to pandas

+ link to README.rst in /docs
CLN: ipython expects None from _repr_html_ to signal no html repr GH5922
Merge pull request #6045 from y-p/PR_ipython_repr_html

CLN: ipython expects None from _repr_html_ to signal no html repr GH5922
DOC: minor doc edits
Add -NaN and -nan to default NA values
Merge pull request #6038 from Bklyn/master

Make -NaN and -nan default NA values
BUG: Regression in .get(None) from 0.12 (GH5652)
Merge pull request #6053 from jreback/get

BUG: Regression in .get(None) from 0.12 (GH5652)
PERF: Speed up pd.to_datetime() by optionally inferring dt format #5490

Given an array of strings that represent datetimes, infer_format=True
will attempt to guess the format of the datetimes, and if it can infer
the format, it will use a faster function to convert/import the
datetimes.  In cases where this speed-up can be used, the function
should be about 10x faster.
PERF: Add infer_datetime_format to read_csv() #5490

This allows read_csv() to attempt to infer the datetime format for any
columns where parse_dates is enabled.  In cases where the datetime
format can be inferred, this should speed up processing datetimes
by ~10x.

Additionally add documentation and benchmarks for read_csv().
TST/DOC: addtl tests and docs for (GH6056)
Merge pull request #6058 from jreback/iloc_segfault

TST/DOC: addtl tests and docs for (GH6056)
BLD: add IPython and sphinx to show_versions #6059
Merge pull request #6064 from y-p/PR_add_deps_show_versions

BLD: add IPython and sphinx to show_versions #6059