(SERVER-151) Add jruby-pool HTTP Admin endpoint

Add a jruby-pool endpoint to the HTTP Admin API that, when given
a DELETE request, will flush the JRuby pool.
(SERVER-151) Address PR feedback

Add comment to jruby-pool-resource similar to that for the
environment-cache-resource. Change resources test directory for
jruby-pool-int-test. Remove unnecessary Accept header from
test of the jruby pool endpoint. Add tests to ensure that the
jruby-pool endpoint tolerates an 'Accept: */*' header and that
it denies access when a cert is not on the whitelist.
(MAINT) Added changelog