Add github_host to Projects.

Generate repository and API links accordingly.
Use the project to locate the API endpoint.
Refactor the commit_status_spec.

Remove some duplication.
Take the trailing / off of the API uri.
Assign :github_host in ProjectsController.
Merge branch 'master' into enterprise
Oops, schema changes from the other branch.
Specs for deriving GitHub host choices.
Choose GitHub hosts to display in projects#edit.
Move #set_hosts to a before_action.

It wasn't firing as an after_action for some reason. We don't actually care
about the `:github_host` on a brand-new Project anyway, though.
Render a select box when there are multiple hosts.
Mention ENTERPRISE_HOST in the README.
Add ENTERPRISE_HOST to the sample `.env`.
Merge branch 'master' into enterprise
Understand multiple ENTERPRISE_HOSTS.
Merge pull request #29 from cobyism/enterprise

GitHub:Enterprise support