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
Correct type name in the doc example
add Hari haran as a contributor
Add get/create blobs
Add more functionality for pull requests
add Trey Tacon as contributor
add support for /markdown endpoint

notably, this also updates client.Do() to handle non-JSON response
bodies by passing an io.Writer as the final argument, v.
split examples into separate directories