Improved plugins, better tests, 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.

tests/dummy_plugin/__init__.py

@@ -1,3 +0,0 @@
-from senpy.plugins import SenpyPlugin
-
-plugin = SenpyPlugin("dummy")

tests/dummy_plugin/dummy.py

@@ -0,0 +1,8 @@
+from senpy.plugins import SentimentPlugin
+from senpy.models import Response
+
+class DummyPlugin(SentimentPlugin):
+    pass
+
+    def analyse(self, *args, **kwargs):
+        return Response()

tests/dummy_plugin/dummy.senpy

@@ -0,0 +1,7 @@
+{
+    "name": "Dummy",
+    "module": "dummy",
+    "description": "I am dummy",
+    "author": "@balkian",
+    "version": "0.1"
+}