Fixes issue #17.

Uses Flask's Request.json for JSON parsing.
Records changes in correct section of changelog.
Uses Request.get_json() instead of Request.json.

This is suggested in the Flask documentation:

  http://flask.pocoo.org/docs/api/#flask.Request.json
Merge pull request #22 from jfinkels/use-request-json

Use Flask's Request.get_json for JSON parsing