Rewrite the profiler

Rewrite the profiler :

- provide more methods
- decouple (un-hardcode) the rendering
- design it so it's easy to write/test a renderer
- 96% of coverage
- documentation updated

It should fix https://github.com/joomla/joomla-framework/issues/62, https://github.com/joomla/joomla-framework/issues/33, https://github.com/joomla/joomla-framework/issues/34