Add tutorial for STAMP

1) Make 'PICRUSt predicted metagenomes' a list of PICRUSt and external tools that can be used with PICRUSt predictions.
2) Move QIIME tutorial to its own tutorial page
3) Add brief explanation and links to PICRUSt's categorize_by_function.py and metagenome_contributions.py
4) Add link and new tutorial for using STAMP with PICRUSt
Remove old prepicrust documentation
Merge branch 'small_doc_fixes' into stamp_tutorial
Improve layout of list of downstream tools
Change documentation layout
Merge pull request #142 from mlangill/stamp_tutorial

Add STAMP tutorial and remove prepicrust documentation