Enable cleaner overriding of TileLayer#getTileUrl

Moved the called to _adjustTilePoint outside of getTileUrl,
and moved the call to _getZoomForUrl into _adjustTilePoint.
This allows getTileUrl to use x, y, z directly without any
calls to private methods.
Merge pull request #1400 from jfirebaugh/getTileUrl

Enable cleaner overriding of TileLayer#getTileUrl