Add Google Guava dependency.

The Google Guava library has grown popular over the last few years. Besides providing common day-to-day boilerplate such as Preconditions and comparator helpers such as Ordering, it also provides a diverse set (pun unintended) of tested useful collections such as Multimaps.

Pulling in a new significantly-sized dependency would be advised only if it is useful. This JIRA is to serve as a list of places in Wicket where the Guava library would prove beneficial, as well as to provide a forum for developer comment on the desirability of adding such a dependency.