adding :through option to replace :nesting option and moving ResourceAuthorization class code into ControllerResource

allow access to classes when using hash conditions since you'll generally want to narrow it down with a database query

removing unauthorized! in favor of authorize! and including more information in AccessDenied exception - closes #40

renaming ActiveRecordAdditions#can method to accessible_by since it flows better and makes more sense

adding caching to current_ability class method, if you're overriding this be sure to add caching there too