Added support for variable-size distance classes in Mantel correlogram.

This pull request adds support for variable-size distance classes (bins) to Mantel correlogram. A new option, ```--variable_size_distance_classes``` is added to ```compare_distance_matrices.py``` to support this, and is disabled by default.

This feature request came up during work on the biogeo paper. Mike Robeson requested that we test Mantel correlogram both with and without this option, as he'd used it previously in [a paper](http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0023742) to ensure that each distance class contains the same number of pairwise distances, though the size (i.e width) of each class may vary.