Added API for broadcast variables.

This is a first step towards realizing the functionality requested in issue #66. 

The broadcast inputs API consists of two aspects:

* Setters: ~~setParameterInput()~~ `setBroadcastVariable()` / `getBroadcastVariable()` functions in `AbstractUdfOperator` and all Operator Builders. Used to broadcast (bind) subplan results to UDF-local variables.
* Getters: ~~getParameterInput()~~ `setBroadcastVariable()` / `getBroadcastVariable()` in `RuntimeContext`. Used within an UDF open() method for parametrization of the UDF with values provided by the plans rooted at the configured parameter inputs.

A sample intended usage of the API can be seen in the modified `KMeansIterativeWithBroadcastVariables` Java example.