Implement mirror executor (issue-5)

The mirror executor will fetch ref updates for repository mirrors.  This
feature is disabled by default and can be enabled by setting
git.enableMirroring=true.  The period between update checks is
configurable, but it is global.  An individual rpeository may not set
it's own update schedule.

Requirements:
1. you must manually clone the repository using native git
   git clone --mirror git://somewhere.com/myrepo.git
2. the "origin" remote must be the mirror source
3. the "origin" repository must be accessible without authentication OR
   the credentials must be embedded in the origin url (not recommended)

Notes:
1. "origin" SSH urls are untested and not likely to work
2. mirrors cloned while Gitblit is running are likely to require
   clearing the gitblit cache (link on the repositories page of an
   administrator account)
3. Gitblit will automatically repair any invalid fetch refspecs with a
   "//" sequence.

Change-Id: I4bbe3fb2df106366ae4c2313596d0fab0dfcac46