Assign context object to Manticore itself (#13)

* Make Manticore behave like a dict; slim callback
* Update all sample scripts to not take a context
* Add a sample script (count_instructions.py)
* We no longer need to close over Manticore state; add callback directly
* Move Manticore's context to a public attribute

examples/script/run_callback.py

@@ -21,7 +21,7 @@ if __name__ == '__main__':
     m = Manticore(None, path, args)
 
     # Trigger an event when PC reaches a certain value
-    def reached_goal(ctx, state):
+    def reached_goal(state):
         cpu = state.cpu
 
         assert cpu.PC == 0x10858