React Component Update

[IhsanE]

This change is to update the current countdown-timer to extend React Component. This is a future proofing solution to aid the use of ES6 classes. Facebook recommends the deprecation of ReactClass, I suggest we follow suit.

Also fixes errors that the current implementation has. The following are listed below.

1. When adding props to component, passing them results in the timer restarting due to colliding logic in componentWillReceiveProps

2. Misuse of the constructor (as per @rloomba implementation)

-- I believe that although this adds the use of es6, it's necessary as an update to the current branch since the current state is basically unusable by any visitors using up to date libraries (most people I would presume).