Remove old precalculated files

Note this is only a soft removal and permanently removing them later should be a priority to help reduce the size of the repository.
Update documentation for new precalculated files

Added doc page with list of all precalculated files from each gg version.

Added extra information to install doc about having to download precalc files.

Added new options for choosing gg version and type of function for the metagenome_prediction tutorial.
Update options for gg version and functional types

Add gg_version 13_5 and make it default.
Add rfam as another type of prediction.
Update version from 0.9.1-dev to 0.9.2
Add picrust/data/* to .gitignore
Remove unneeded import statement
Update automated script documentation.
Update metagenome_contributions.py to load new precalc files
Fix download links for precalculated files.

Found out GitHub's release files have some type of file size limit.
It wouldn't allow the upload of a >100mb file. Change to using dropbox direct download link.
Update version from 0.9.2 to 0.9.2-dev
Fix bug caused by version renaming in test_ace.py
Update gg_version option
Raise exception if not all OTUs in user table are found in precalculated file.
Merge branch 'master' of github.com:picrust/picrust
Update script usage for metagenome_contributions.py
Fix bug when no ids are given to load in convert_precalc_to_biom
Add option parallel_predict_traits.py where predictions can be added to previously generated file.
Update script usage for normalize_by_copy_number.py
Abstract out default filename handling into its own functions.  This allows reuse for loading tables of variances
Tests for variance combination fns