Creates user-bar many to many relationship through bar_gig_memberships

* Creates owner-owned bar relationship
* Generates bar_gig_relationship model
* Implements user partial
* Adds delegate methods to comply with law of demeter
* Requires bar_id and user_id in bar_gig_membership table
* Adds validations for selected fields
Merge pull request #10 from mtdowd/bar-membership

Creates user-bar many to many relationship through bar_gig_memberships