zoomlevel, resolution and scale handling:
Added getters for resolution and scales to Layer (http://dev.openlayers.org/apidocs/files/OpenLayers/Layer-js.html).
XYZ layer can get an offset for zoomlevels and a range of zoomlevels can be set.
This allows to use XYZ sources, which do not contain all zoomlevels or should be limited and corresponds with http://dev.openlayers.org/apidocs/files/OpenLayers/Layer/XYZ-js.html.
Merge pull request #5 from sebasbaumh/zoomlevelresolution

zoomlevel, resolution and scale handling: