adding quadkey as file cache option

This pull request gives you the possibility to use MapProxy as a way to generate a static quadkey cache. These static cache are needed for certain software packages on microsoft based surface tables.

One discussion point (both for client and cache) for me is how MapProxy handles the (0,0,0) xyz tile.
Here: https://github.com/mapproxy/mapproxy/issues/49 somebody notes issues with a client requesting this (0,0,0) tile.
But if I understand correct from: http://msdn.microsoft.com/en-us/library/bb259689.aspx there is actually not 0,0,0 tile in quadkey. The first 'level' there is is level 1.

So maybe we should throw some exception when requesting 0,0,0 ?

By the way: current implementation is working correct if requested as wms etc.