Python API documentation generation tool Code

- If the dot executable is not found, issue an error, and don't try to
generate further graphs.

- Fixed bug that causes crash when we can't determine the canonical name
for a metaclass.
- Fixed bug in callgraph rendering.

- Added missing global SCRWIDTH

- Fixed silly typo

- Fixed silly typo

- Added --graph-image-format option

- Changed the usage for to_html() and to_latex() methods -- they now
accept a directory, rather than an image filename (and a url in
the case of to_html). This is in preparation for adding an option
to specify the image format for graphs in html output.

- Changed the usage for to_html() and to_latex() methods -- they now
accept a directory, rather than an image filename (and a url in
the case of to_html). This is in preparation for adding an option
to specify the image format for graphs in html output.

- DotGraph's write() render() and html() methods now auto-select the
output format (gif, png, etc) based on the filename. Default=gif.

- Moved PYTHONPATH statement up