Message callbacks should be called in the main thread

[aalex]

When we receive some data, the callback is called in the web socket channel's thread. It can lead to serious issues when mixed with other non thread-safe calls.

[aalex]

I had severe issues when using this library within a Twisted project. I tried to handle this non-thread-safe code like this: http://twistedmatrix.com/documents/current/core/howto/threading.html - but it didn't quite work.