Added an option to use websockets without masking

Motivation:
The requirement for the masking of frames and for checks of correct
masking in the websocket specifiation have a large impact on performance.
While it is mandatory for browsers to use masking there are other
applications (like IPC protocols) that want to user websocket framing and proxy-traversing
characteristics without the overhead of masking. The websocket standard
also mentions that the requirement for mask verification on server side
might be dropped in future.

Modifications:
- Added an optional parameter allowMaskMismatch for the websocket decoder
that allows a server to also accept unmasked frames (and clients to accept
masked frames).
- Allowed to set this option through the websocket handshaker
constructors as well as the websocket client and server handlers.
The public API for existing components doesn't change, it will be
forwarded to functions which implicetly set masking as required in the
specification.
- For websocket clients an additional parameter is added that allows to
disable the masking of frames that are sent by the client.

Result:
This update gives netty users the ability to create and use completely
unmasked websocket connections in addition to the normal masked channels
that the standard describes.