Embed XPI in DLL, and extract & use that by default, to simplify the API

This fixes #4. I think you're right; embedding it in the DLL is definitely the right way to go. This adds a `AddExtension(ffProfile)` method that uses the XPI extracted from the DLL directly, without you having to specify a path.

I think with this the previous method (`AddExtension(profile, path)`) doesn't have any real use cases at all anymore. For now I've just obsoleted it, to keep backward compat.