Removed unsupported functionality

Removed subprotocol and extension getter functionality, as this
is currently not supported by QtWebSockets. I left the backend
implementation intact (in the _p files), for future reference.

Change-Id: Ia00739f6f225cd557ceb10b40c83fc81d98aae0b
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Reviewed-by: Kurt Pattyn <pattyn.kurt@gmail.com>
Update documentation

Clarified which versions are supported and that subprotocols
and extensions are not supported. Fixed some typos.

Change-Id: I99ef2fbbf74c287bea7d1b7a07881bf0542135ff
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>