Example program

This is a short example program, along with a unit test.

It defines a small, simple string expression language that shows how to build a simple "domain specific language" in Python using Parsimonious.

I created an `parsimonious/examples/` directory - not sure if this is the best place to put it. Also I'm not sure where the best place to put the unit test for it so that it will get run with the other tests, so it's in the examples directory for now. Let me know if there is a better place.