Promote Guice extension

This is the bare minimum to promote the Guice extension from Restlet incubator to module.

It still lacks tests; the tests (examples, really) in the incubator don't fit the pattern used in other extensions, so I've eliminated them.

At the moment the module.xml file only endorses this extension for jse and jee, but it's possible that the android, gae, and osgi editions will compile with no problems.

I don't know the wikiUrl for this extension, so I've commented it out.