Support for dashed lines in various patterns

This is based on a [protovis modification by Jamie Love](http://groups.google.com/group/protovis/browse_thread/thread/c71d20e9700b32a4/1f415b7252f655da).

Example:

    line do
      stroke_dasharray('9,5,3')
      # ...
    end

Seems to work fine. Updated specs. Had to add an rspec patch to debug, though.