Correlations

Commit adds ability to correlate numeric metadata with feature abundance values as well as do paired t tests.
It's not an error in any of the options passed in the script, so I thought at `ValueError` was more appropriate. If this error is raised, it's because the biom table or mapping file didn't contain enough valid (i.e. float convertible) data to run a correlation. Happy to change it if the appropriate response is still an `option_parser.error` but it seemed inconsistent with my understanding of that call at the time.