Convert HTML Purifier to a single filter with implementations provided by hook_htmlpurifier_info()

Let's get this in front of some other eyes to see if these changes make sense to anyone else. I've looked at it way to long.

http://drupalcode.org/project/htmlpurifier.git/commit/279674.

I'm getting the cart before the horse a little. Here's the rational for these changes.

• No need for a warning message to only select one of the two htmlpurifier filters.
• No longer need to "hack" the module to insert a custom configuration. These are provided from implementations of hook_htmlpurifier_info
• Distribution authors can easily write their own filter configuration to include in their distro without the need to use Features.
• The full configuration including filter tips can be written in code rather than stored in database entries. This helps with dev/stage/prod situations.