Node rectangles now use more style attributes from the node's meta-data

This is a minor step in the direction of fully configurable node shapes.

In addition to the already supported fill and href attributes, the
following additional (style) attributes are now supported:
- width and height
- stroke, 
- stroke-width, 
- stroke—dasharray, 
- rx, ry

Changes:
- Added a Boolean argument “adding_node” to addLabel to differentiate 
  the cases of adding a node or an edge label. It can be argued that it 
  is better to split addLabel in two separate functions. I have not 
  done that to make the changes as small as possible.
  
- In the body of addLabel: added code to set the various style    
  attributes when they are defined in the node’s meta-data.