Instantiation

Hey,

I've did a major rewrite of your really nice countdown timer. It should also work with the current specification.

Example for illustration:

```
var timer = $('#timer').final_countdown({
    seconds: {
        borderColor: '#fff',
        borderWidth: '10'
    },
    minutes: {
        borderColor: '#fff',
        borderWidth: '10'
    },
    hours: {
        borderColor: '#fff',
        borderWidth: '10'
    }
});

// Set times (start, end, now)
timer.set(new Date.now().getTime() - (15 * 1000), new Date.now().getTime() + (15 * 1000), new Date.now().getTime());

// Start the timer
timer.start();

// Stop the timer
timer.stop();
```
The improvements in detail:
- returns instance of countdown object
- counters aren't bind anymore to an id
- redefines (set) the timers dynamically
- start/stop timers
- arbitrary definition of days, hours, minutes and seconds

Improvements:
- "use strict" directive
- JSLinted

Solves:
PragmaticMates/jquery-final-countdown#5