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.
Closes#28 for python 2.
Apparently, process pools are not contexts in python 2.7.
On the other hand, in py2 you cannot pickle instance methods, so
you have to implement Pool tasks as independent functions.
Closes#28
Added:
* Async test (still missing one that includes the IOLoop)
* Async plugin under tests. To manually try async functionalities:
```
senpy -f tests/
```
