modify create_chart and take_screenshot

- create_chart can accept page template as argument, defaulting to rChart2.html
- take_screenshot whitens images using imagemagick convert.
modify render_template to accept file or string as argument.
add notes to xcharts examples.
modify code to make use of refactored version of render_template.
fix bug in setLib that set chartDiv to NULL.
Merge remote-tracking branch 'upstream/dev' into dev
fix chartDiv.
fix link to twitter boostrap prettify theme.
Merge remote-tracking branch 'upstream/dev' into dev