Added limit for days in the future to convert

I added a future conversion limit to the settings, such that you may set a variable similar to allowFuture that will cause timeago to convert future timestamps up to that many days away to words, otherwise it will print a date string.

For example, 
```javascript
	$.timeago.settings.allowFuture = true
	$.timeago.settings.daysInFuture = 3
	$("time.timeago").timeago()
```
will convert any dates past three days in the future to a full date string. Today is April 23rd, so for April 27th this would print "Saturday, April 27, 2013" instead of "4 days from now".