Multiple modifications

Renamed the script ocs.py to test_group_significance.py.
Renamed the library ocs.py to otu_significance.py (both group tests and correlation tests will use it)
Placed relavent functions in qiime.utils.
Placed tests for those functions in tests/test_utils.py
Rewrote Mann-Whitney U test to make it faster/cleaner and to prevent behavior when using mw_boot where it would stochastically error. The reason it stochastically errored was because if it would randomly choose two samples which were all the same, then the ranks would all be equal, and the denominator of the modified t statistic would drop to zero. It now returns nan for a pvalue in this case.
Added a large number of tests to pycogent_backports/test.py.
Rewrote the subsampling routine for the mc_t_two_sample test to avoid stochastic errors that were caused by differently ordered vectors.