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.
I had thought about that a bit, and it's got enough idiosyncratic bits that it'd be pretty hard to do well, so I'm not going to try. When I first opened that file in my editor, that's what I tried to do, BTW, but it was... tricky. :wink: