IronPythonScriptCatalog - Fixes an early design error and allowed better interop 

Hi,

This is a bit of a big PR, but I think it fixes the early design error of using the ComposableParts directly to manage imports and exports.

What I've done is create an IronPythonScriptCatalog that's similar to a TypeCatalog, except that it reads from an ironpython script file. It allows for better interop with the rest of the MEF machinery, and I think clears up the path for some improvements like the ones mentioned on #3 .

It's still a bit barebones, but I'd love a review and sanity check. Ideally I'd be able to improve on it to include creation policies and custom metadata.