Try to ensure that sloth uses the corresponding module

This also makes it possible to run sloth without installing or setting
PYTHONPATH.

sloth/bin/sloth

@@ -1,5 +1,7 @@
 #!/usr/bin/env python
-import sys
+import os, sys
+from os.path import dirname, realpath
+sys.path.insert(1, dirname(dirname(dirname(realpath( __file__ )))))
 from PyQt4.QtGui import QApplication
 from sloth.core.labeltool import LabelTool
 from sloth import APP_NAME, ORGANIZATION_NAME, ORGANIZATION_DOMAIN