Switch to Gorilla websockets package

[davecgh]

There are some issues with the go.net websockets package such as the lack of support for generating ping websocket protocol messages to help keep the connection alive and the requirement to cast between strings and []byte (causing extra allocations and mem copies) to determine the frame type.

In practice, these issues don't prevent the current code from working, but the recent btcrpcclient changes already make use of the Gorilla websocket package and work is underway to also convert btcwallet to use the package as well. This will help keep the code consistent as well as make it slightly more efficient.