This commit fixes handling of branches when using both V3 and V2. The
changes include:
* proper definition of relationships that reflect V3 structure, so for
example build belongs to a branch
* setting up inverse records for some of the relationships. without
doing that Ember Data can handle relationships in a surprising way,
for example if the same record is referenced in 2 places in a
belongsTo relationship, Ember Data will remove one of the references
without proper inverse definitions
* we need to add id when extracting branch as a relationship. Ember
Data expects all of the relationships to have an id
* lastly, we need to mimic the structure of the V3 API in V2 payloads,
so for a build payload I'm now creating a branch record
One thing that is not standard here is a serializer for branch, which
uses @href as id. At this point branches don't have ids and ember-data
needs one, so using @href is the easiest way.
