Schema migration framework for OpenChange DB MySQL Backend

Features:

   * It provides migration indexed by a number
   * It can apply/unapply migrations to a defined version
   * It can list migrations applied or not

Added two new options to `openchange_provision` command:

 * `list-migrations` to list the migrations along with the applied date
 * `migration-target-version` to apply/unapply to a specific migration version

It is highly inspired by @blaxter and @exekias code along with Django migrations code.