Adding functionality to the loop option

Very short fix. So far the ```loop``` option in animator.js had no effect, so it was always enabled. With this if it's set to ```false``` it won't loop. This will be useful for animations where showing all steps clearly is important (i.e. maps with a small number of points).

Ref issue #14 

cc @javisantana 
do we want to stop at time = 0 ? I think the loop should stop at the last frame