Add specification-based tests via FsCheck

### Goal
Test corner cases of pretty printing F# ASTs using FsCheck.

### References
http://jasani.org/2008/01/03/testing-haskell-with-quickcheck/
http://lstephen.wordpress.com/2007/07/29/parsec-parser-testing-with-quickcheck/
http://book.realworldhaskell.org/read/testing-and-quality-assurance.html

### Tasks

 - [ ] Create test generators for F# expressions
 - [ ] Create test generators for F# type definition (this part in the code base seems more buggy)
 - [ ] Use shrinkers (if possible)