Common solution for loading custom jars

Building on top of good work of @colinsurprenant in #1233, 
logstash can now almost propose a common solution for loading jars present under vendor/jar
I'm working on jdbc related plugin (input/filter) and this patch would permit me to load the required jar in one line in the register method like:
````
def register
  LogStash::Environment.load_jars!(@driver_jar)
  #Cannot use driver manager (https://jira.codehaus.org/browse/JRUBY-7174)
  @driver = Java::JavaClass.for_name(@driver_class).ruby_class.new
end