(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 think it's important to have these cases covered for each of the endpoints.  To cut down on the code duplication a bit, we could wrap the body of these tests inside of a `doseq` over the unique portion at the end of the URI or something along the same lines.  @KevinCorcoran what do you think?