zoom function returns wrong level for levels < 3

[jgoizueta]

I've been checking the !scale_denominator! values that Mapnik provides for each zoom level and the zoom function here returns the actual level plus one for levels 0, 1 & 2.

I've been testing this fix for cartodb-postgresql (which borrowd the zoom code) and works for our purposes there (matching zoom level for a given !scale_denominator!).

cc @CartoDB/stamen can you check this is also a issue here?