Class based styling

Changes the previous concept of inline styling to class based styling. The class definitions are placed into a style element, added at the top of the SVG structure. This preserves the aspect of keeping the SVG copy&paste-able without losing the styling. It also allows usage of external CSS style sections to define the classes used in the graphs.
Limitations: 
* This only works for styling of nodes, but this is also the only case where classes where available in the syntax.
* Not implemented for sequence diagrams

Resolves issue #24.