Use <g> to simplify SVG and fix styling. Add name attribute file mechanism.

Place rect and text into a g block.
Replace the duplicated rect and text onmouseover with a single g onmouseover.
This reduces the size of the SVG file. It also fixes the loss of the
black border when mousing over the text.

An external file can be used to specify attributes for individual
functions. So, for example, functions can be given links and colors.
default target=_top else links will open within SVG <object>
Merge pull request #11 from timbunce/linking

Use <g> to simplify SVG and fix styling. Make functions clickable etc.