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Add library and some helper classes.
I'd like to discuss before adding this library, as it seems heavy weight. Maybe a white list would be better (ie. trust content from google.com, feedburner.com ?).
In terms of size: HTML Purifier 4.2.0 Lite would add 350K to the size of the .zip file; unzipped it's an extra 1.9 MB.
Thought: add the piwik helper classes but use a mock object if htmlpurifier isn't installed
In , add stub support for HTMLPurifier library