[rtorrent] HTTP-RPC support

This adds the ability to specify URIs for connections to HTTP-RPC rutorrent endpoints. Also made some changes to ```connect()``` as ```rpc_url``` is not used in scgi and httprpc. This resolves #2699 (already closed though).

The following formats are supported, it's basically just the URL for rutorrent with ```httprpc://``` instead of ```http://```.

    httprpc://localhost/rutorrent
    httprpc+https://localhost/rutorrent

(assuming rutorrent is on ```localhost``` and setup at ```/rutorrent``` and not ```/```)