Add tab-delim output format in categorize_by_function.py

Addresses #85
Merge pull request #86 from mlangill/issue85

Add tab-delim output format in categorize_by_function.py
Add tab-delimited version of tutorial OTU table.
Added picrust_test_data directory

This is a directory to store example input and output from PICRUSt scripts. This data is used for testing of script interfaces, and to provide examples of the files used and generated by PICRUSt.
Merge pull request #90 from gregcaporaso/add_test_data_dir

Added picrust_test_data directory
Add links to Galaxy instance of PICRUSt in docs.
Merge branch 'master' of github.com:picrust/picrust
Store and link to release version within github (instead of beast)