Split out `projector.utils`

[Mr0grog]

The utils in projector should probably be split into a different module. Most of them are related to projecting between 4326 <--> 900913, which could easily be split out on their own.

Also, given the comment in

nodetiles-core/lib/projector.js

Line 17 in 56f1643

// TODO: just use https://github.com/mapbox/node-sphericalmercator/blob/master/sphericalmercator.js

, we should look and see whether it would make sense to just use node-sphericalmercator (one question: feasible for browsers? see #21).