Update to version 0.4.1

*** IMPORTANT NOTE ***

0.4.X is the last minor version to support the Hixie protocol. 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: add 'include/' sub-dir to source distribution and to
  installed packaged.