0.5.0-beta7

• Makes main element classnames including placeholder configurable and prefixed (NB! .placeholder class is now .wysihtml5-placeholder)
• Changes API by grouping class names under classNames option
• Functions passed to check_attribute methods have now second argument node name
• Functions passed to check_attribute methods can now decide if empty argument value will delete the attribute or keep empty value ("" vs null)
• Fixes an incorrect array iteration that could cause errors if Array prototype is extended