initial support for creating and sending nested bundles

I'd not only like to receive, but also create nested bundles with liblo, so this patch adds support for creating and sending nested bundles.

There's just one new function for the API: lo_bundle_add_bundle

This is a proposal for a simple initial patch which makes use of the existing infrastructure introducing the concept of bundle elements (which can be either a message or an other bundle).

However, by introducing nested bundles, we introduce all the problems of handling potential cyclic references in the bundle trees and the serialisation step will run havok (like a bundle in a bundle which has the parent bundle as an element).

That's something that's not yet cared for by this patch (because it's not a trivial problem) and would need some clever approach to the problem. Any ideas?