Modestmaps

- Fixes bug where Layer.draw would try to draw even if not attached to a map.

- Adds `.enable()` and `.disable()` methods to layers, and
`.enableLayer()`/`.enableLayerAt()` and `.disableLayer()`/`.disableLayerAt()`
to the map object.

- Fixes bug in the MM.MapProvider.sourceCoordinate where tiles containing
areas within the tile limits were not loaded at lower zoom levels

- Adds named layers: a third parameter to MM.Layer and MM.TemplatedLayer
that sets the `.name` property of a layer. The layer name can be used
with the new `map.getLayer()` method, which returns the first layer
matching that name, and `map.removeLayer()`, which now accepts either the
literal layer object or the layer name as its one argument.
- Tests have moved from `/test/browser` to just `/test`

- Explicitly set the size of tiles to the `tileSize` parameter in the Map
object in order to support tiles that are not 256x256

- Fixes a bug in the TouchHandler leading to inaccurate movement
when on scalable devices like iPhones and not at native screen resolution.