More flexible logging configuration, JSONEvent support

There are a bunch of things people might be interested in doing
wrt to riemann logging configuration. I tried to capture the most
common use cases, leaving more advanced configuration to people
willing to just drop in their log4j.properties file.

This code does not modify the existing behavior and thus will
not introduce inconsistencies for already configured riemann
instances.

It does add the following:

* ability to disable console logging
* ability to log to multiple files
* a choice of 3 layouts: the existing one (:riemann), :json-event(-v1)

The layout used for console output is specified with :console-layout
and defaults to :riemann, like today. Specifying a different layout here
can help people using runit to supervise riemann, since runnit expects
console logging.

The :json-event and :json-event-v1 layouts are useful for people shipping
riemann logs to logstash or elasticsearch and seems like a wide enough
standard (and a light enough dependency) to make the cut of riemann core
dependencies.