Implemented only one Interval on a page.

This commit will fix jquery.timeago's issue to have an interval
for each timeago element on a page.
By doing this it also removes destroyed elements automatically.

As such, this commit fixes rmm5t/jquery-timeago#96 and
rmm5t/jquery-timeago#90.

Please share your comments. My code might not be the most idiomatic jQuery plugin code ever - please let me know what could be done better in your opinion, as well as let me know if there are any bugs.