Added broadcast variables support to the runtime.

This is a first step towards realizing the functionality requested in issue #66 in the runtime, building upon the API proposed in PR #383.

Broadcast variables are realized as additional outputs attached to a `RegularPactTask`. Convenience methods for accessing the configuration information for broadcast input setup was added to the `TaskConfig`.

An integrated test case for with a simple hardwired JobGraph can be found in `BoradcastVarsNepheleITCase`.