Added code for the Content API

With this code it should be possible to tick the ‘Content’ box in issue
#37. The code is written after reviewing PR #64 and so is (very)
loosely based on that PR. So big thanks for the work already done by
@octavore! I did however had some different insights about a few fixes
and as the PR hasn’t moved for the last 5 months, I thought to just
rewrite and PR and I send in a new PR hoping to come to a merge…
add Sander van Harmelen as contributor