Reworking the plugin interface. Retrieving injection modules from plugins.

Removing legacy plugin loader.
Declaring base MessageInput classes abstract.
Extracting GELFParser from GELFProcessor to be reused in input plugins.
Using multibindings instead of registries to manager input & filter classes.
Minimizing jars in package task.
Merge branch 'master' of github.com:Graylog2/graylog2-server