HelpBlock now implements HasHTML allowing put html tags inside widget's ...

...UiBinder xmls.


This PR aims to solve a issue related by [@rabbitfr](https://github.com/rabbitfr) originated by https://github.com/gwtbootstrap/gwt-bootstrap/pull/431.


[@rabbitfr](https://github.com/rabbitfr), please confirm if your problem has been solved.