There's no reason for builtins to have a leading '_'

The Policy._builtins dictionary doesn't need a leading '_', so let's
drop it.
Allow the context class to be overridden

One of the first steps of Policy.evaluate() is to allocate a
PolicyContext object.  Providing the ability to override the object
built may be necessary for some functions decorated with
@want_context--in order to initialize context attributes, for
instance--so this adds a "Policy.context_class" variable, which can
be changed on instances.
Bump version to 0.3.