Fixed #61. @api_guard takes content-type as argument.

If the content-type does not match with the argument, @api_guard will
abort with HTTP status code 415 media unsupported. A few backend
requests calls have to be modiifed to support this strict content-type
requirement.

Improvement to be made? We probably can subclass app = Flask(__name__)
and use method-based. For example, @app.content_type('text/plain') and
customize @app.route so that by default it checks backend key.