Add exam Prelude cheat sheet as pdf and tex.

For the exam in EDAN40, there is a Prelude cheat sheet available, on the website only as a simple .hs file, I thought it was a bit easier to read/print as a PDF-file. 

Source: http://fileadmin.cs.lth.se/cs/Education/EDAN40/exams/preludefns.hs