Index changes by external issue tracking systems

Parse out external references from footer lines in the commit message
based on configuration in gerrit.config.  Connect the change with
the external tracking ids and tracking system in a new table in
the DB, making the tracking ids searchable by tr:<id>.

Bug: issue 124
Change-Id: I3ddada57240040f27329f5f26f1f8e99e94f1469
15 files changed