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.
consistency between input spans and getCoordinates

[NEW] Map class get's a getCoordinates() method. It chekcs the self.Reverse
    property for establishing how to order start/end values in the returned
    series. _Annotatble.getCoordinates() method now uses this method on Map.
[FIXED] On reverse strand _Feature, getCoordinates() did not return a value
    that was usable. Instead of [(end, start), ..] you got [(start, end), ..].
    The latter could not be used to recreate the _Feature. This is now fixed.
Merge pull request #17 from jrrideout/mc_t_two_sample_fix

Bug fixes for t_two_sample and mc_t_two_sample.
Docs now point to GitHub issue tracker.
Merge pull request #19 from jrrideout/docs_github_port_fix

Docs now point to GitHub issue tracker.