Basic support for rest controllers coming from plugins.

This is done using a static REST resource that looks up the resource
by Path. This is not a good solution because it basically does what
Jersey (or any other JAX-RS implementation) should do.
Unfortunately adding the REST resources coming from plugins as child
resources to the ResourceConfig object did not work properly.
Adding registered plugin REST resources to Jersey's ResourceConfig.
Merge branch 'pluggableRestControllrs'

This features allows plugins to bring their own JAX-RS resources, register
them and have them available in their own namespace based on a static prefix
(currently "/plugins/") and the package name of the plugin's binding module.
Below this, resource mappings are based solely on resource definitions.
Merge branch 'master' of github.com:Graylog2/graylog2-server