High level client code generation

[acrisci]

In addition to introspection, add an option to create high level clients with code generation.

This may involve extending the introspection format with doc comments (gtkdoc has a spec for this).

Advantages:

• You don't need to rely on introspection
• You don't need to include introspection xml in your project or get it dynamically
• Better code completion
• Better support for enum types and property types
• You can include common interfaces in the library

[garyvdm]

I'm working on this, and have initial promising work. I'll hopefully be able to share more soon.