Remove the HttpRequest::get() method.

I'm in the process of adding support for cancelling HTTP requests, and I need to really make sure no one is messing around with the evhttp_request, otherwise we're all going to lose our marbles.
... then these could go and we'd be able to stop/reduce libevent C functions from leaking out of the libevent wrapper (JsonObject c'tor aside, but that could potentially be sorted