use fork of php-mf2 until https://github.com/indieweb/php-mf2/pull/83 is merged
see http://microformats.org/wiki/microformats2-parsing-issues#ignore_u-camelCase_properties for background
allow only a basic set of tags, and remove any non-mf2 classes closes #2
sanitize the HTML returned in the content property. allows a common set of HTML tags. for #2