Rewrote Korma chapter, made code all work "as-is", added some new content showing off query composition.

And since I believe in showing one's notes, I followed along with the instructions of the chapter (even using a Mac that didn't previously have PostgreSQL installed) and used unit tests in the example "learn-korma" project to make certain all the code examples worked.

https://gist.github.com/bitemyapp/7078907