GMap enhancements for compatibility with Wicket Ajax

This commit includes the following changes:

* The HeaderContributor provided to GMap is now only added
if it is nonnull. This is useful when employing the GMap in
an ajax modal window.

* The visibility of the get... JavaScript Methods has been
altered to protected, because they might be needed by classes
extending GMap.

* The Google-Maps API url now includes an encoded '&' character,
because IE's produce an XML parsing error when used in an wicket
ajax response.