Adding sharedClasspath and extraClasspath support for local containers.

I needed access to the sharedClasspath and extraClasspath properties on [CargoTask](http://cargo.codehaus.org/maven-site/cargo-extensions/apidocs/org/codehaus/cargo/ant/CargoTask.html).  They appear to be only available for local containers (see [SpawnedContainer](http://cargo.codehaus.org/maven-site/cargo-core/cargo-core-api/cargo-core-api-container/apidocs/org/codehaus/cargo/container/internal/SpawnedContainer.html)). It's working for me locally, and I enhanced the README to document them. Continued thanks for the good work!