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/count_instructions.py

@@ -0,0 +1,29 @@
+
+import sys
+from manticore import Manticore
+
+'''
+Count the number of emulated instructions.
+
+This example uses the context property of the Manticore object to store data
+that's updated by the hook function. Manticore.context is needed to properly
+share data when running with multiple worker processes.
+'''
+
+if __name__ == '__main__':
+    if len(sys.argv) < 2:
+        sys.stderr.write("Usage: %s [binary]\n"%(sys.argv[0],))
+        sys.exit(2)
+
+    m = Manticore(sys.argv[1])
+    m.workers = 3
+    m.context['count'] = 0
+
+    def explore(state):
+        m.context['count'] += 1
+
+    m.add_hook(None, explore)
+
+    m.start()
+
+    print "Executed ", m.context['count'], " instructions."