Add ProxySocket for opening remote socket

The existing proxy functions set up local listeners, but some programs
are better served by having access to the original socket.

`ProxySocket` returns a `net.Conn` which is piped to the sprite proxy
WebSocket. The interface is modelled after `net.Dial`, and is generic
for the type of socket being opened. This makes backwards compatibility
easier if we ever add new proxy types (Unix, UDP, etc.)

Author: jbellerb