[core/engine] Add module loading logic

Allow the engine to load modules from the bumblebee/modules/ directory.

see #23

bumblebee/modules/test.py

@@ -0,0 +1,11 @@
+# pylint: disable=C0111,R0903
+
+"""Test module"""
+
+import bumblebee.engine
+
+class Module(bumblebee.engine.Module):
+    def __init__(self, engine):
+        super(Module, self).__init__(engine)
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4