Automatic import of existing repositories on file system

When migrating existing repositories to Bonobo Git Server, it was previously difficult to automatically add the repositories in the Bonobo database.  With these changes, the application will now scan the repositories directory when it starts up.  For each sub-directory, it will create a repository entry.

Additionally, if a repository's directory gets deleted on the file system (but still exists in Bonobo), the text "(Missing)" will appear next to its name.

Please let me know if this needs any changes to be acceptable for the main project.

Also note that I implemented this change off of the 2.0.1 tag, then merged into master.