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
Added:
* Async test (still missing one that includes the IOLoop)
* Async plugin under tests. To manually try async functionalities:
```
senpy -f tests/
```
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.
