Break dependency of ApplicationContext on Pyro

ApplicationContext imports PyroProxyFactory so that it can tag it as a type to avoid. This is requiring users to install Pyro in order to use the IoC container. Replacing it with some alternative strategy would remove this dependency.

==From an posting on the mailing list ==
Some problems here:

• springpython.context depends on pyro just because it imports
  PyroProxyFactory to add it in types_to_avoid, but this seems a bit of
  overkill.