property :extension, String, :default => "png"
property :max_zoom, Fixnum, :default => 18
property :url_aliases, [String, Array], :default => []
-property :revision, Fixnum, :default => 1
+property :revision, Fixnum, :default => 0
property :overlay, [TrueClass, FalseClass], :default => false
property :default_layer, [TrueClass, FalseClass], :default => false