Improve Content Node Binder interface

To get rid of a bit of legacy and make the usage of the CNB more intuitive, these could be nice improvements (from an email by Woonsan):

I think we can improve the following:

• ContentNodeBinder could be parameterized like ContentNodeBinder<T
  extends HippoBean>. So no casting is necessary.
• Maybe we can introduce a new interface, ContentBeanBinder which has
  only #bind(Node) like you mentioned. So, wpm can check whether or not
  the interface implemented in addition.

In the second case the code in the bean could be much simpler:

@Override
public boolean bind(Object content, javax.jcr.Node node) throws ContentNodeBindingException {
    try {
        node.setProperty("myhippoproject:username", getUsername());
    } catch (Exception e) {
        log.error("Unable to bind the content to the JCR Node" + e.getMessage(), e);
        throw new ContentNodeBindingException(e);
    }
    return true;
}