Extensible ModulePrefs, better visitor pattern

This patch improves on the ModulePrefs system by cleaning up the visitor objects and adding a new visitor ExtraElementsVisitor

The ExtraElementsVisitor accumulates any unknown tags and stashes them in a Multimap based on the key.  Currently the list of visitors is still hard coded, but we're very close to making it Guice injectable.

[johnfargo, 2009-09-22 21:59:58]

One nit, but the majority concern I have is in the overall result of this
approach. Seems to me this encourages ad hoc modification of the spec, leading
to inconsistencies and incompatibility between gadgets. Why not use the existing
<Requires>/<Optional> syntax? It's limited, but typically can get the job done,
using a known pattern.

http://codereview.appspot.com/121064/diff/1/2
File java/gadgets/src/main/java/org/apache/shindig/gadgets/spec/ModulePrefs.java
(right):

http://codereview.appspot.com/121064/diff/1/2#newcode26
Line 26: import org.w3c.dom.*;
please expand these wildcards, verbose as it may be.