Add a global property controlling whether or not GUI functions are enabled
Remove a pointless try/catch, add a bool use_gui parameter to br_initialize

Qt prefers calling abort to throwing exceptions, so there's no use trying to
recover from a failed attempt to create a QApplication. Add an optional bool
use_gui paramter to br_intialize in case people want to set that value without
going through the command line arguments.
Merge pull request #42 from biometrics/gui_switch

Add a global property controlling whether or not GUI functions are enabled