git-cola: defer module imports so that PyQt4 can be found on Mac OS X

The sys.path setup happens in cola.app so we must defer importing any
other cola modules until after it has been imported.

Closes #221

Reported-by: Alexander Kozienko <a.s.kozienko@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>