Correlations

Commit adds ability to correlate numeric metadata with feature abundance values as well as do paired t tests.
`option_parser.error` should be used to report problems to users, regardless of whether these were caused by a problem with the options or a problem in the semantic content of the inputed files. The reason is that users should not try to read a python traceback as that is usually "intimidating". Also, with #1794 the messages will be more succinct and even point users to a [help page](http://help.qiime.org).