Update to version 0.4.0

*** IMPORTANT NOTE ***

This is the last minor version to support the Hixie protocol. I may do
bug fixes (e.g. 0.4.1) but 0.5.0 will drop the Hixie protocol support.
The latest released version of all major browsers (and web-socket-js)
support the IETF 6455 protocol.

- Other changes: updating TODO and release process notes.