PEP8+Better JSON-LD support

* 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)

tests/dummy_plugin/dummy.py

@@ -1,6 +1,8 @@
 from senpy.plugins import SentimentPlugin
 from senpy.models import Response
 
+
 class DummyPlugin(SentimentPlugin):
+
     def analyse(self, *args, **kwargs):
-        return Response()
+        return Response()