Merge Cargo's curl-rust branch back in

A summary of the high level modifications:

* The libcurl library is now bundled
* A build script is now included (soon to land in Cargo) for building this library (defaults to detecting it on the system)
* SSL support is now explicit and `ca_path` and `ca_cert` now have defaults when enabled and necessary.
Can't you write the same without `let _ =`?