Corrected error in Mantel test

My previous method shuffled the distance vector rather than the
distance matrix for the sake of efficiency. It turns out that this is
wrong and exaggerates the Z-scores. The issue is corrected in this
commit. Added a separate Mantel test module (MantelTest.py). For more
info, see my blog post here:
http://www.jonwcarr.net/blog/2014/9/19/a-guide-to-the-mantel-test-for-li
nguists
Separate options for confidence levels

Plots can now optionally show the normal confidence levels and the
Bonferroni-corrected confidence levels independently of each other.