refactored CssSchema to distinguish between a schema, a collection of property...

refactored CssSchema to distinguish between a schema, a collection of property filters, and the properties themselves which are now instances of an inner class.  Added code to the policy builder to allow a styling policy to be created with a custom schema.

git-svn-id: http://owasp-java-html-sanitizer.googlecode.com/svn/trunk@197 ad8eed46-c659-4a31-e19d-951d88f54425