Ant task for Calabash

This adds (and even documents) an Ant task for running Calabash.

You can use this task to process:
* A single input file to produce a single output file
* A set of input files, processed one at a time, to produce a set of output files
* Multiple input files as the input to one XProc input port processed to produce a single output file
* Any of the above with additional input ports to each of which are applied one or more input files whose file names may be either fixed or mapped from the name(s) of the current main input file(s)
* Any of the above with additional output ports whose file names may be either fixed or mapped from the name(s) of the current main input file(s)

You can also specify options and parameters to be used by the pipeline.

This doesn't yet handle everything you can do from the Calabash command line or config file, but if it proves useful, then more can be added later.

The tests in `test/build-antunit.xml` do require AntUnit to run.

The license viewpoint is murky since it is influenced by the Ant `<xslt>` task code, among other reference sources, but it could never become part of Apache Ant itself because Calabash is licensed under GPL.