Add minimal OSGi metadata as per #768

[cowtowncoder]

So, as per #768 we may want to add minimal OSGi metadata here, and eventually across all format backends plus probably jackson-databind. But starting with basics.

Also note that if this works, 3.0 (master) uses bit different bindings (TokenStreamFactory that JsonFactory implements) -- but for 2.x we'll stick to JsonFactory as being the base class every format backend implements.

[cowtowncoder]

@chrisr3 Is this what you meant in #768? Not sure how to test goodness (or lack thereof).

[chrisr3]

Yes, I think that looks right. AFAIK this is the only jackson-core class with a META-INF/services entry, and jackson-core doesn't explicitly use ServiceLoader.load() either so this should be the only annotation that needs adding.