Add hook decorator (#28)

* Add m.add_hook test

* Add @m.hook test

* Add `hook` decorator for convenience

* Update readme and examples

* Update run_callback

* Improve `add_hook` docstring

expound on callback structure

* Rm debug print

* Improve docstring

examples/script/count_instructions.py

@@ -19,11 +19,10 @@ if __name__ == '__main__':
     m.workers = 3
     m.context['count'] = 0
 
+    @m.hook(None)
     def explore(state):
         m.context['count'] += 1
 
-    m.add_hook(None, explore)
-
     m.run()
 
     print "Executed ", m.context['count'], " instructions."