Also:

* added drone-ci integration: tests for py2.7 and py3

There are also some slight changes to the JSON schemas and the use of
JSON-LD.

Fixes #2

* The API has also changed, there are new parameters to send the
context as part of the headers.
* Improved tests
* PEP8 compliance (despite the line about gevent)

Moved from Yapsy again (it is not flexible enough), now we use a
custom solution.
The activation and deactivation of plugins is asynchronous, so
that plugins that take a long time don't interfere with the rest.