tile size moved to a constant
using a render cache when the map is not being animated

when the map has no animation (for example for static datasets) when the user pans the map it feels pretty slow because torque needs to render all the points again

with this change the tiles are rendered and backed to an offscreen canvas so when the map moves torque does not need to render every point, just flip the image for every tile

Code to clean cache is also added, when the user zooms or a tile is loaded the caches are cleared
updated news
cache variables for looping
Merge pull request #44 from CartoDB/test_back_per_tile

using a render cache when the map is not being animated