[engine] Fix interval configuration

Until now, manually specifying an interval did not work, as a cast to
float was missing. Now, it's possible to specify an update interval in
seconds via "-p interval=<interval>"

fixes #54

bumblebee/engine.py

@@ -158,7 +158,7 @@ class Engine(object):
             self._output.flush()
             self._output.end()
             if self.running():
-                self.input.wait(self._config.get("interval", 1))
+                self.input.wait(float(self._config.get("interval", 1)))
 
         self._output.stop()
         self.input.stop()