JERSEY-2112 Updated documentation to make it explicit that resources mus...

Updated documentation to make it explicit that resources must be Spring components in order for Spring proxying to work, and also created test cases to make sure Spring proxying actually works as expected.

This is just a partial fix for the discussion in JERSEY-2112. The original report seems to be the fact that it is not obvious that the resources must be annotated `@Component` (or similar). However, there is *another* bug that makes it impossible to use this on filters and providers - this is not handled by this pull request.
Fixed.