Enable generating JAX-RS documentation from interface.

[mwinkels]

This patch allows for generating jax-rs documentation based on an annotated interface instead of an implementing class.

[pdeschen]

Any chance of getting this merged an release?

[FroMage]

Unfortunately I don't have time to keep maintaining jax-doclets. If anybody wants to take the responsability of merging this and testing it and making a new release, I'll give you commit access.