Add support for simple dynamic types.

Add support for simple dynamic types.

Defining types in runtime-loaded modules requires a slightly different
get_type implementation, and the type registration is done explicitly
using a different API.  It's an extra pile of GObject boilerplate which
is a good fit for GOB to generate.

Add a (dynamic) flag which can be put on any class definition which
causes gob to spit out these alternate implementations.