TST: use python 2.6-compatible testing statements
BLD: test_perf.py gc disable can exhaust memory (where are the cycles?)
ENH: HDFStore now auto creates data_columns if they are specified in min_itemisize on appends

DOC: created String Columns section, added cookbook example
Merge pull request #3357 from jreback/hdf_fix

ENH: HDFStore now auto creates data_columns if they are specified in min_itemsize
BLD: Add 2to3 caching, move the build cache logic into use_build_cache.py
DOC: update CONTRIBUTING.md
Merge pull request #3358 from y-p/build_cache_v2

BLD: Add 2to3 caching, move the build cache logic into use_build_cache.py
CLN: DataFrame move doc from __init__ to cls

This commit moves the doc string to a more common location.
CLN: treat missing cases of __doc__ under __init__
DOC: add class docstrings in various places
PERF: get_numeric_data now a bit faster

CLN: cleaned up take in tseries/resample and core/internals to use interenals take

     (which wasn't using the block level take)
Merge pull request #3359 from jreback/perf_numeric

PERF: get_numeric_data now a bit faster
BUG: MultiIndex summary now inserts an ellipsis GH3348
BLD: test_perf learned --stats and --quiet
BUG: pivot_table bug when rows is unspecified #3361