Migrate to using libcurl for now instead of hyper

Unfortunately the move to rust-native-tls has had some hiccups on Windows and
Hyper also currently doesn't support features like HTTP proxies (but support's
coming soon!). For now this temporarily moves over to using libcurl to weed out
these problems until we get to a point where we're hyper's features are more
filled out. This also somewhat aligns with Cargo as well which is using libcurl
currently.

This does not use the `curl` crate on crates.io because it doesn't support
streaming downloads. Instead I've got an in-progress rewrite which is just
binding much more directly to libcurl's APIs (e.g. exposing the callback
functions directly). This branch is in a git repo currently and I hope to
upstream it to curl-rust soon as well.