Auto version for packages and UI

This is the first approach at having fully automated version number generation for the packaged versions of Tribler.
Now by just running Tribler/Main/Build/update_version_from_git.py before starting the packaging script for every platform, all the necessary files/fields will get updated. The resulting packages will be something like:
Tribler-6.2.0-36-g80f0eea.dmg where 6.2.0 is extracted from the latest release tag on the branch we are building, 36 are the number of commits between the last tag and the current point and g80f0eea the actual abbreviated commit hash.

The same applies to the windows installer and .deb packages.

There are test jobs for each platform at

http://jenkins.tribler.org/job/Build-Tribler_OSX_next_auto_version/ 
http://jenkins.tribler.org/job/Build-Tribler_Ubuntu-12-10_amd64_next_auto_version/
http://jenkins.tribler.org/job/Build-Tribler_Ubuntu-12-10_next_auto_version/
http://jenkins.tribler.org/job/Build-Tribler_WinXP32_next_auto_version/

When running directly from the git repo, Tribler will show 6.2.0-GIT as version.