BUG: NaN values not converted to Stata missing values

Stata does not correctly handle NaNs, and so these must be replaced with Stata
missing values (. by default).  The fix checks floating point columns for nan
and replaces these with the Stata numeric code for (.).  One of the code paths
which writes files correctly handled this case, and this last-minute check was
removed.

The write_index option was also being ignored by omission. This has been fixed
and numerous tests which were not correct have been fixed.

Also contains some additional tests which were uncovered edges cases related to
fix.
Merge pull request #6685 from bashtage/stata-world-indicators

BUG: NaN values not converted to Stata missing values (GH6684)
BUG: Bug in resample with extra bins when using an evenly divisible frequency (GH4076)
Merge pull request #6690 from jreback/resample_bin

BUG: Bug in resample with extra bins when using an evenly divisible freq (GH4076)
Update documentation on visualization to mention using pandas objects directly with matplotlib and the automatically installed date and time formatter.
Merge pull request #6660 from agijsberts/docs-plot-with-mpl

DOC: Mention date and time formatting available throughout MPL
Deprecate cols in to_csv, to_excel, drop_duplicates, and duplicated.  Use decorator.  Update docs and unit tests. [fix #6645, fix#6680]
PERF: dataframe.from_records: read into list w/ itertools.islice when specifying nrows #6700
DOC: add to docstring of core/groupby/Grouper
BLD: use numpy 1.8.1 in 2.7 build
     use scipy 0.13.3 in 3.3. build
BLD: move sql libs up in requirements to facilitate building wheels
dataframe bar plot can now accept align keyword
Merge pull request #6691 from sinhrks/bar_center_pr

ENH/VIS: Dataframe bar plot can now handle align keyword properly
EHN/FIX: Add na_position parameter to DataFrame.sort. Fixes GH3917
TST: Skip mergesort test if `np.argsort` raises TypeError on any mergesort.
Merge pull request #5231 from unutbu/nan-sort

EHN/FIX: Add na_last parameter to DataFrame.sort. Fixes GH3917
BUG: Bug in consistency of groupby aggregation when passing a custom function (GH6715)
COMPAT: change Series.sort to use na_position rather than deprecated na_last
BUG: shift now converts named axis to numer. fixes #6724
TST: test named shift with named axis
Merge pull request #6725 from dalejung/shift_named_axis

Shift named axis
Merge pull request #6718 from jreback/groupby_agg

BUG: Bug in consistency of groupby aggregation when passing a custom function (GH6715)
TST: Added tests for pairwise behaviour in rolling moments.

Added a test for rolling_cov_pairwise similar to the test for
rolling_corr_pairwise.

Added tests for ewmcov_pairwise and ewmcorr_pairwise.

Added test for expanding_cov_pairwise based on the test for
expanding_corr_pairwise.