Added reset & fix memory leak when unmount parent

[alesanderlopez]

Counter.js

• added method: [reset] to reset the counter again to start.
• added method: [componentWillUnmount] to stop the counter and fix memory leak when Unmount parent view or change view in react-native-router-flux.
• updated method: [animate] for stop the loop when the componentWillUnmount was executed.

Readme.md

• added example to call reset from parent view.