Example slider

It is an example of slider maps.
Ticked related: https://github.com/CartoDB/cartodb.js/issues/150
The example compares USA states (1790 vs 1890). Thanks @iriberri datasets :)
If you know another datasets that could be better, please, let me know.
Also, the arrow images are not great (I am a poor designer :( ) and the middle image (comparer) is by default. 

Please, @saleiva @javisantana @xavijam , could you review it, please?