(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.
It seems like the addition of this test case obviates the need for the `basic-flush-test` from which it was derived ... if we're going to add this test case, I don't see any reason in keeping `basic-flush-test` around.