first changes for 0.20.0

everthing is now prepared to accept messages from all source with all kinds
of data. we are no longer limited to log data. host is source now. everything
except source and message is optional and additional fields don't exist anymore
because now everything except the two standard fields is an additional field.
severity is renamed to level, underscore prefix for additional fields is gone.
also introducing the new stream rule model: it does not matter on which
fields you apply a rule now. the rules only differ on matcher type. supported:
regex, exact, greater, smaller. we also have the first REST APIs - the new
web interface is already using them. great times are coming!
work on APIs for new web interface
forgot to commit changes
fix warnings
fix droelf warnings
Merge pull request #7 from kroepke/020

fix warnings
Add subdirectory for graylog2-plugin-interfaces
Merge history of graylog2-plugin-interfaces/020 into plugin-merge
Remove unused binary dependencies from lib/ directory
Merge graylog2-plugin-interfaces with graylog2-server sources