WIP: Fix for issue #34

@matallo in #34 we have this example failure:

http://wip.gfw-apis.appspot.com/datasets/forma.shp?begin=2006-06-08&end=2013-12-08&iso=bra

Basically what's happening is the response Shapefile ZIP is too big for App Engine URL fetch (32 meg limit). I can detect when this happens and either:

1. Have the API redirect to the underlying CartoDB URL request
2. Have the API return the underlying CartoDB URL string itself

What do you think? Preferences either way on the client side?