Modifications in preparation for final merge.

Removed otu_gradient_correlation.py as we have determined we are not yet ocmfortable with the functionality even though the tests pass.
Changed the ChangeLog.md to reflect the new script.
Added otu_category_significance.py back in with a deprecation message.
Resolved an error @lkursell located

If biom didn't contain taxonomy information (e.g. a summarized table) a TypeError wouldn't be caught.
Actually resolved