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/sym.py

@@ -29,7 +29,7 @@ if __name__ == '__main__':
     else:
         target = (0x400a83, 'EBX')
 
-    def entered_func(ctx, state):
+    def entered_func(state):
         '''For ARM, Make R4 symbolic at 0x1082c, as r4 is used in a branch right
            after.
         '''