example improvements

[Douglas Squirrel]

The calculator example we wrote yesterday got a cosmetic overhaul:

1. You can now run the calculator from the command line. Each keypress is handled independently (like a real calculator) using the JLine library http://jline.sourceforge.net/. I enjoyed adding the library without having to download it and decide where it should go (thanks, MavenCentral!)

2. The calculator acceptance test has comments in it that should help people figure out what it does and how it works.

 

Also, updated the README to refer to the now-commented test.