add comparisons operator to the Period object and fixes issue 2781
move the Period tests in test_period.py and add assertRaises tests for comparisons
use a function factory for comparisons of Period objects
ENH: assert_X_equal with check_less_precise should apply to index/columns too
DOC: provide an axis alises example in the cookbook.rst
BLD: fix path handling, and possible import surprises in test_perf
TST: add test for mom._flex_binary_moment blowing the stack GH3155
BUG: _flex_binary_moment should validate it's arguments' type
TST: added rolling_corr test with cener=True GH3155

Conflicts:
	pandas/stats/tests/test_moments.py
BUG: rolling_corr should pass center=True to it's delegates
DOC: update RELEASE.rst
BUG: fixed issues where passing an axis of 'index' would fail (as its expecting an axis number)
CLN: change to_csv 'legacy' keyword to engine=='python', consistent with c_parser
Merge pull request #3110 from jreback/aliases

BUG: GH3109 fixed issues where passing an axis of 'columns' would fail
Merge pull request #3177 from jreback/alias_cook

DOC: provide an axis alises example in the cookbook.rst
BLD: move perf_HEAD functionality into test_perf. -H arg
BLD: fix path handling in test_perf
BLD: test_perf will read BUILD_CACHE_DIR from .build_cache_dir if it exists
BLD: test_perf, add -n, number of vb runs to average results over
BLD: test_perf, add -N, run HEAD vbenchmarks multiple times seperately