Modifications to resolve documentation errors

I removed:
	doc/scripts/otu_category_significance.rst

I modified to include ```group_significance.py``` instead of ```otu_category_significance.py```:
	scripts/core_diversity_analyses.rst
	tutorials/open_reference_illumina_processing.rst
Added the otu_category_significance.rst file back

Now a pointer to use group_significance.py
Merge pull request #1282 from wdwvt1/group_significance

Looks good to me!!