Implement __contains__ ('x' in Choices('x')) for Choices objects.

In Choices, `_choice_dict` appears to be a definitive location of all
internal/DB representations, so it seems the best target for finding
out if the given item is part of the sequences.

model_utils/choices.py

@@ -76,3 +76,7 @@ class Choices(object):
     def __repr__(self):
         return '%s(%s)' % (self.__class__.__name__,
                           ', '.join(("%s" % repr(i) for i in self._full)))
+
+    def __contains__(self, item):
+        if item in self._choice_dict.values():
+            return True