This patch adds support for GitLab API v4 while maintaining backwards

compatability with v3. When we don't know the API version of the host,

we will determine it by optimistically assuming v4 and then falling back

to v3.
Additionally, we no longer use the user's password to retrieve an API

token from GitLab. Newer versions (9+) remove the /session API and

require users to generate a token per application in their profile

settings. We now accept this API token directly with instructions how to

create it on new and old versions of GitLab.
Some other minor changes were made:

We now log full stack traces from unexpected Exceptions in the

  Repository Form
We now have commit summaries for post commit review for older versions

  of GitLab that don't include the entire commit message
Support for nested groups in newer versions of GitLab

Linked a gitlab.com nested group repository (e.g. group foo/bar, repo

  baz) and was able to do post-commit review.
Linked a GitLab 7 group and user repository and was able to do

  post-commit review for both.
Ran unit tests.