Moved WebDAV functionality in an appmod and locking server in a runmod

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](http://www.webdav.org/neon/litmus/). All tests are passed, except the ones that require unsupported properties. The test shows this as a fail while returning 404 should be sufficient.