Replacing request.el with our own fork.

The primary change is that we now get proper JSON response data in error
responses. This is critical for properly interacting with YCMD.
I see your point. I guess my hope is that something like `f` - which I consider a step forward for elisp APIs - will eventually be integrated into emacs proper. For now, though, I'm happy to remove the dependency, and we can add it back if we start doing lots of path manipulation for some reason.