add documentation of exhtml directive to yaws.tex (by tjeerd)
Move WebDAV to an appmod, the locking server to a runmod (tjeerd)

All WebDAV functionality is moved to an appmod so the extension methods and
additional response headers are not part of yaws_server.erl and yaws.erl
anymore. Also the locking server is now a runmod so it is not started by
the supervisor and can be used only when needed.

Additionally the 100-Continue code is moved so it can be returned to all
requests that contain a body.

The WebDAV functionality is tested against the litmus test. All tests are
passed, except the ones that require unsupported properties. The test shows
this as a fail while returning 404 should be sufficient.