New script for getting fixed bugs list

The script must be executed within a git repository
by simply calling
getbugs.py previous current

where both previous and current are git references
like tags, local branches, remote branches, hashes.
For example:
 $ getbugs.py ovirt-engine-3.4.3 origin/ovirt-engine-3.5

It output on stderr the progress and on stdout
the lines to be added to release notes.

Typical use case is getting the list of fixed bugs
from previous release while releasing a new version.

Change-Id: Ib178735cd17df91090c158f7e0ad3cbe36595000
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>