Clean up setSize API

[tmcw]

map.setSize accepts either (x, y) or ({x: 0, y: 0}) as arguments, and then it passes [{x: 0, y: 0}] to its callback. I don't grok any strong reasons why three types of representing size are necessary here - I think we should choose one (probably the size-object), and just go with that.

[RandomEtc]

It's a bit weird that it's the only thing that allows x,y or a point, it's true! Though I see no reason to enforce that the supplied arg should be a Point when any x/y Object will do.

I'd be OK losing support for setSize(x,y) in favor of consistency with other APIs, but it would be our first API breaking change. Perhaps we should make a list of these and see if we can make a clean break to v1.0.0? I've made a start on this on the wiki: https://github.com/stamen/modestmaps-js/wiki/v1.0.0-API-proposals

Regarding the callback, we should make sure that map.dimensions is always a com.modestmaps.Point, even if a { x: 0, y: 0 } literal is passed in, and that the callback receives MM.Point accordingly.