Add rust-http as a submodule

Conflicts:
	.gitmodules
Add rust-http to the build and begin using it to resolve http urls
Use https for rust-http submodule
auto merge of #863 : brson/servo/http, r=metajack

This adds rust-http as a submodule and wires it into the resource loader for http urls. It *does not* do DNS resolution yet, sadly, since Rust doesn't support it yet. That's what I'll be working on next.