Some gobjectization

Hi Paolo,

I spent some time during the week end to make the public API gobject-like and generate GIR bindings.

It's still not fully GObject like, and it's probably what I should tackle next (return pointers directly, not by arguments, use GError etc..)

I don't think I break more tests.

I would also like to simplify LibmsiRecord & LibmsiField, since it's quite a simple code easy to refactor (simple API), and could probably just wrap GValueArray.. What do you think?