WIP: Initial version of script engine for R

It uses the Rserve library for interprocess communication between
Java and R.

Here is an example annotated R script:

  # @Parameter(name = label, type = string)
  # @Parameter(name = version, type = string, output = true)
  version = paste("[", label, "] ", R.version.string, sep = "")

Outstanding issues:

  * Reconcile with updated scripting framework on master
  * Deploy Rserve JARs to ImageJ Maven repository
  * Fix bug where DisplayPostprocessor thinks output is null
  * Support multiple output values
  * Compare this Rserve-based solution to one using rJava
  * Eliminate FIXME blocks
    - Reduce redundancy of "bindings" code across scripting languages
    - Implement get/set variable methods properly for R
  * Add support for Dataset, DatasetView, ImageDisplay