PyQt4 frontend, frontends option, debian packages

Pull request has everything from https://github.com/sommer/loxodo/pull/23
with a little improvements:
Separate debian packages for wx and pyqt4 versions (they still has the same code, the only difference is the command line arguments, this can be improved later).
Option for switching frontend is available in settings in the both frontends, also it possible to select frontend by using command line argument.
List of available frontends is dynamic and depends on the installed requirements (wxwidgets, pyqt4).