WICKET-5771 Ability to escape resource bundle messages added with
wicket:message
WICKET-5771 Ability to escape resource bundle messages added with wicket:message

Non functional improvements:
fix javadoc errors, cache and reuse local variable, extract constants for the attributes, Java 7 diamonds
WICKET-5772 set attached after load, so failuer of load() will not keep null value
WICKET-5774 UrlRenderer should render full and absolute urls in their canonical form