Add a release script

Currently releases are done via the manual procedure on
http://code.google.com/p/ganeti/wiki/ReleaseProcess

, but that is not
very reliable, and breaks for rc releases. The 1.2.9/2.0.5/2.1.0~rc2
releases were done with this new script that eases the process.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>