Add subsection about Variables. Use minted for haskell code instead of listings

* I'm using the development version of minted since I want to use the
  command \mintinline. This requires either install this package
  manually or just placing the file "minted.sty" in the same directory
  as the report. "minted.sty" can be found here:

  - https://raw.github.com/gpoore/minted/master/source/minted.sty

  Minted also requires an external python executable (pygmentize).
  This executable can for instance be installed using the
  "python-pygments" package on debian/ubuntu and "python2-pygments" on
  arch linux.
update report Makefile for minted
Change some wording.

* Don't use the term constructor or context since these concepts has
  not been formally introduced yet. Change usages into "available
  function in the user API".