The goal is to help reduce generated binary size for that only use
the low-level wrapper.

Packages net and net/url use fmt package, making it not possible to
remove the relatively large fmt dependency from the websocket package.
Package net is only used to implement the net.Conn interface.

Separate the low-level plain wrapper into a different package from
the high-level net.Conn wrapper, making it possible for users to
import only what they need at a more granular level.

Updates #18.
Updates #19.