Fix display paths on windows

Cached display paths must always use url forward slashes.  This fixes a
windows specific bug.
Handle webapp contexts.
Full and overridden asset paths.

* Tag includes full scheme://host:port by default.
* Tag includes context path by default.
* Tag now allows overriding host with webasset.host property to allow
  cname cdn setups.
Web asset url needs to be a property, not message.

* Allow overriding the web asset url throuh the web asset manager.
  Needs to be property, not localized message.
* Don't specify port if overriding web asset url.
Refactor remove WebAssetUtils HttpServletRequest.

* Added README with extensive information.
* Added WebAssetRequest and WebAssetRequestFilter classes that use Threadlocal
  to hold request ids.  No more importing HttpServletRequest.
* Added tld to resources/META-INF so tld will be auto included in projects.