Copied style to style_werkblad

Start work on a separate style for werkblad files.  For question
environments a lot of work is already done by the exam class, but that
class does not play nice with our styles.  I'll try to fix that.
Fixed some typos in CONTRIBUTING
Style file now works with exam class.
Broke out common style code
Merge branch 'layout' into elementaire_deeltjes
Customized werkblad style

We can now print a specified number of lines, yay!
Werkblad ED now uses new werkblad style

Numbering of questions is automatic. Use of \paragraph, \\ line breaks,
manual ruling inside center environments and \bigskip is no longer
necessary.