Fix reading the WebSocket handshake HTTP reply
in case it does not arrive at once.
Strengthen the unpredictability of the masking key

Use the whole uuid by XOR-ing each 4 byte of it together
to create the 32-bit key that is needed.
[EnginioBackendConnection] Add support for large payloads.
[EnginioBackendConnection] Handle PING and PONG messages

Additionally make it possible to send PING messages and receive
notification about PONG's from the server.
Test ping operations in notifications tests.
EnginioBackendConnection should not have its own EnginioClient

This way we can share the EnginioClient in the EnginioModel where
we will make use of notifications in the firs place.
Merge pull request #192 from bbandix/websocket

Websocket improvements