Starter for exercise W5

After yesterdays exercises I have made a basic parser framework with way too many comments.

There is however still some holes for parsing the exact expressions. 

Please consult the Parsec documentation for further details and inspiration. Look especially at `chainl` and `chainr` combinators

https://hackage.haskell.org/package/parsec-3.1.17.0/docs/Text-ParserCombinators-Parsec.html