Draft API for parameter inputs for UDFs (i.e. broadcast vars).

This is a first step towards realizing the functionality requested in issue #66. 

The parameter inputs API consists of two aspects:

* Setters: `setParameterInput()` and `getParameterInputs()` functions in `AbstractUdfOperator` and all Operator Builders. Used to configure inputs (plans) for UDF parametrization.
* Getters: `getParameterInput()` 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 `KMeansIterativeWithParameterInputs` Java example.