Improve agents diagram.

- Fold each agent name after every 20+ characters.

- Choose the rounded box as node shape.

- Signify disabled agents with gray color, including arrows.

- Use a shorter node name to shorten code that often exceeds the limit
  of Google Chart API when an agent with a long name has lots of sources
  and receivers.

- Use a smaller font size, and specify Helvetica as font family in SVG
  output to match the rest of the site contents.
Set the link target of nodes to _blank.
Merge pull request #413 from knu/improve_diagram

Improve agents diagram.