package: point "xmlhttprequest" to our LearnBoost fork

Which currently includes support for the `agent` option for passing a custom
`http.Agent`.
util: make the request() helper accept an "options" object
util: add jsdoc comment
polling-xhr: pass the `agent` option to the XMLHttpRequest constructor
transports: pass the options to the `util.request()` function

Probably not strictly necessary in this case...

In fact, I can't seem to find this `poller()` function being used anywhere
in the codebase in the first place! It only seems to be invoked in the tests...