Add irmin.unix dependency

Signed-off-by: John Else <john.else@citrix.com>
Add module Db_git

Signed-off-by: John Else <john.else@citrix.com>
Add DB converter

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #1 from johnelse/dbconvert

First steps towards creating a xapi db converter