Add pwmantel for pairwise mantel tests

Core algorithm is working. No tests, useful error messages, or coaxing of distance matrix IDs to a compatible order. p-values need to be formatted for the correct number of digits. Still not sure whether to return a DataFrame or some sort of results object that stores a DataFrame with extra metadata. Going with DataFrame for now as it's simpler.
Port make_compatible_distance_matrices; add basic tests
Format p-values as strings w/ correct num decimals
Add tests for N/A p-values and too few permutations
Add DissimilarityMatrix.index method
Add DissimilarityMatrix.__contains__
Add DissimilarityMatrix.filter
Add 4x4 test case to filter tests

Fixes #186.
Add unit test for reordered dms
Add unit test for nonmatching IDs
Rewrite make_compatilble... and add tests for lookup
Add error messages and error path unit tests
Add additional tests
Add docstring for pwmantel
Add mention of pwmantel to changelog
Simplify logic in DistanceMatrix.__contains__

Thanks @gregcaporaso
Clarify `strict` description in docstring

Based on @gregcaporaso's suggestion
Clarify `dms` parameter description

Thanks @gregcaporaso
Add skbio.math.stats.misc module

Moved p_value_to_str there, thanks for suggesting @gregcaporaso!
Add sphinx rst files for new stats modules