(SERVER-151) Add jruby-pool endpoint

This PR adds a jruby-pool endpoint to the Admin HTTP API. Making a DELETE request against this endpoint allows the user to flush the JRuby Pool.

This should be merged before https://github.com/puppetlabs/puppet-server/pull/320.
I don't like duplicating these test cases for every endpoint in the admin API ... but I can't come up with an alternative approach that I like any better.  @camlow325 - any ideas?