Embed Gorilla WebSocket

[FZambia]

Unfortunately Gorilla Websocket is poorly maintained these days. Some examples:

• [BUG] 1.5.1 uses log.Printf gorilla/websocket#880
• [BUG] 1.5.1: use of closed network connection, 1.5.0 is fine gorilla/websocket#877
• Do not handle network error in SetCloseHandler() gorilla/websocket#863
• golang.org/x/net is vendored in branch 1.5.1 gorilla/websocket#874
• Module path not updated to github.com/gorilla/websocket/v2 and new tag does not follow semantic versioning gorilla/websocket#860

We have enough expertise here to fork it and maintain for our own needs.

The idea here is to not fork though but embed to internal dir, and possibly only necessary parts of Gorilla WebSocket. The reason of embedding instead of having separate module – I don't want other developers to rely on websocket package API which I suppose may evolve to fit Centrifuge needs and we better keep it internal.

[FZambia]

Done, this is unfortunate thing in general, but we need to move forward.