Update build instructions for app.

As the lazybones app is no longer a simple Groovy script, I have updated the
build instructions in the README. To be honest, the build is still very simple
but now developers will at least know the entry point for the code, making it
easier to contribute.
Add functional test harness.

This is the first stage of issue #5. It incorporates the running of functional
tests into the build and provides an abstract class to make it easy to run
lazybones as an external process and check its output.
Clarify that templates can be contributed as packages.
Add a little blurb to 'help' output.