Bug fixes for t_two_sample and mc_t_two_sample.

Fixed bug in cogent.maths.stats.test.t_two_sample that returned a t-statistic with the wrong sign if the first distribution contained multiple values and the second distribution contained only a single value (this was validated against R's t.test). Unit test has been updated accordingly.

cogent.maths.stats.test.mc_t_two_sample now handles distributions (as well as permuted distributions) without variance (in the past it would crash with an uninformative error message in these cases). For example, if comparing [1,1,2] to [2,2,1], a possible permutation is [1,1,1] vs [2,2,2], which would return a t-statistic of None. This caused mc_t_two_sample to crash. This was validated against R's Deducer::perm.t.test.

To fix these bugs, added new optional argument none_on_zero_variance to cogent.maths.stats.test.t_two_sample and cogent.maths.stats.test.t_one_observation that is True by default to not modify the previous behavior or change the interface of the two functions.

Updated and added unit tests for these special cases.
Added check for t == 0 when negating t.

Added corresponding unit test.
Merge pull request #17 from jrrideout/mc_t_two_sample_fix

Bug fixes for t_two_sample and mc_t_two_sample.