Constraints and the like
Merge branch 'master' of https://github.com/biocore/american-gut-web into minor_db_improvements
Some unique constraints
Merge pull request #233 from adamrp/minor_db_improvements

Minor db improvements
fixes #232
Add link between surveys and barcodes
Merge pull request #236 from adamrp/survey_to_barcodes

Add link between surveys and barcodes
Merge pull request #235 from wasade/issue_232

fixes #232