Fix typo
Extend instead of replace myo::Hub.

This allows you to call waitForMyo() and get a non-null pointer if a myo
is connected. However, poses recognized by the myo will not trigger an
onPose event.

This is a step towards resolving issue #2, but it would be nice if poses
from the myo could also trigger poses.
Add ability to add any number of listeners.

Each listener's onPose is called when a pose is simulated.