Remove git repo in installation

We're in a gem now, woo!  (and we have been for nearly a month)
Support overriding of association builder

Adds Metadata::Ingest::Form#association_class to allow for injecting
the class that is used to create associations.  Also splits the
association creation into its own method to allow for easier overriding
of the entire creation process if swapping the class isn't enough.
Add manual error support to associations