Added url protocol handler

Following up on JBoss support addressed in this [PR](https://github.com/webjars/webjars-locator/pull/19) and this [issue](https://github.com/webjars/webjars-locator/issues/44), here's my proposal:

* A `UrlProtocolHander` interface to which the creation of asset paths can be delegated.
* The implementations are discovered via ServiceLoader.
* There are two default implementations: `FileUrlProtocolHander` and `JarUrlProtocolHandler`. These are straightforward copy & pastes of the code that was in WebJarAssetLocator.
* Support for other protocols can be added in the standard ServiceLoader manner.

This allows a separate webjars-locator-jboss project to be created, with no impact on the main project.

\cc @LeRiton