Adds PCoA.py. Allows users to create a 2D or 3D PCoA plot.
Resolves issue with matplotlib of opening more than 20 plots at once.

The script now just closes each plot after saving plot image to file.
Adds restrict_repset.py to scripts.

Allows users to create a new OTU representative set file from a filtered BIOM table.
For example, the QIIME compute_core_microbiome.py script creates multiple filtered
BIOM tables for each core percentage (50 to 100). A user can take one of those
tables and their original rep set file for the full dataset and automatically generate a new
rep set with only those OTUs found in the BIOM table they supplied. The new rep set
could then be used to generate a new phylogenetic tree which has many downstream
uses.
Merge branch 'master' of github.com:smdabdoub/qiime-tools