Adds support for future dates (Fixes #2)

Adds support for future-dates/timestamps, rather than displaying "Just now" for any date in the past. Fixes #2.

I have used the following for testing (although clearly this is time sensitive);

```javascript
var ft = require('../fancy-timestamp');
console.log('14/11/2014 10:00:00: ', ft(1415959200));
console.log('15/11/2014 10:00:00: ', ft(1416045600));
console.log('16/11/2014 10:00:00: ', ft(1416132000));
console.log('17/11/2014 10:00:00: ', ft(1416218400));
console.log('21/12/2014 13:00:00: ', ft(1419166800));
console.log('01/01/2015 00:00:01: ', ft(1420070401));
console.log('25/12/2013 09:00:00: ', ft(1387962000));
console.log('25/12/2013 09:00:00: ', ft(new Date(2013, 11, 25, 9).getTime(), true));
```