Added better transitions for #timer-description and #set-default.

- Further generalized .fade-class, .fade-in, and .fade-out. .fade-in and .fade-out are the same at their base, but can be extended to have different effects for each transition (e.g. the Start/Stop button fades out and moves up while it fades in and comes in from the bottom).
	* Adjusted other CSS to incorporate this generalization.
- Added Time.prototype.delete to handle clearing the function interval and getting rid of the Time object.
- Added logic to checkIfStarted() to allow stopping the timer by calling stopTimer().
	* stopTimer() deletes the current time object and calls doStopToStart(), which handles the transition effects.
	* This can not handle the case in which the timer has already reached 0:00 and must be reset. This must still be implemented.
	* checkIfStarted() has also been moved up near the top of the code to be closer to the functions that it calls.
- Renamed doControlTransition() to doStartToStop().
	* doStartToStop() and doStopToStart() are pretty similar and could probably be joined into one function to avoid writing the same stuff twice (even though I already did).