Pass along &ua and &uip parameters.

This PR adds the `ua` and `uip` query string parameters, as described in [this Measurement Protocol thread](https://groups.google.com/forum/#!msg/google-analytics-measurement-protocol/8TAp7_I1uTk/CIJtk3W3L-UJ). We had some trouble getting the client's IP address to be passed along properly using Universal Analytics, and this fixed it. For classical GA there should be no side-effects.