Fixed py2 problems and other improvements

We've changed the way plugins are activated, and removed the notion of
deactivated plugins.
Now plugins activate asynchronously.
When calling a plugin, it will be activated if it wasn't, and the call will wait
for the plugin to be fully activated.

requirements.txt

@@ -2,6 +2,7 @@ Flask>=0.10.1
 requests>=2.4.1
 tornado>=4.4.3
 PyLD>=0.6.5
+nltk
 six
 future
 jsonschema