Force the usage of the Racc Gem.

This ensures that we're loading the Gem and not the standard library version.
Implemented a basic SAX API.

This API is a little bit dodgy (similar to Nokogiri's API) due to the use of
separate parser and handler classes. This is done to ensure that the return
values of callback methods (e.g. on_element) aren't used by Racc for building
AST trees. This also ensures that whatever variables are set by the handler
don't conflict with any variables of the parser.

This fixes #42.
README example on using the SAX parser.
Updated the changelog.
Namespace YARD Rake tasks under "doc".
Corrected YARD argument name.
Added SAX parsing to the list of parsing features.
Added headings + examples to the changelog.
Added changelog note about the date format.