Better XSS protection

* Add HTMLPurifier library (LGPL)
* Add helper functions to html helper
* Set default encoding header to UTF-8
* Make sure the doctype is the same everywhere (admin/members/frontend)
* Remove use of strip_tags() and htmlspecialchars()
* Replace vanilla htmlentities with html::escape() - make sure no one forgets the UTF-8
* Remove _csv_text() fn - no longer used and was using strip_tags()

Conflicts:

	application/libraries/VideoEmbed.php
	application/libraries/XMLImporter.php
	themes/default/views/header.php
	themes/default/views/reports/detail.php
	themes/default/views/reports/list.php
html::clean() : Make allowed tags and iframes configurable

Conflicts:

	application/config/config.template.php
Fix double escaping and missing library after merge of #1056