Group lines by layer so they can be properly depth sorted..

See #2153 

This groups the strokes and oneway markers by layer into separate svg groups so that the ways render more correctly.

![screenshot 2014-06-06 16 37 58](https://cloud.githubusercontent.com/assets/38784/3205719/799afdba-edba-11e3-8808-f1216364db5f.png)
![screenshot 2014-06-06 16 08 24](https://cloud.githubusercontent.com/assets/38784/3205637/47bd8660-edb9-11e3-95a0-4b8fc6a3b43e.png)
![screenshot 2014-06-06 16 24 17](https://cloud.githubusercontent.com/assets/38784/3205639/49fc3c1e-edb9-11e3-8299-221efa43f4c2.png)


It looks cool!  But there are some issues with this still.  Simple edits or moves seem to cause exit() selector to wipe out a whole layer. Moving the map will bring things back, but I think my D3 code has problems.