[add-art dev] Banner art update code checked in

[Ethan Ham]

Assuming I didn't do anything bone-headed, the changes for having the banner
art updated is all checked in.

Among the changes are two files that I added to the server folder:

* image_set.xml
This file must be located at an URL that corresponds with the urlCheckXML
variable in addart.js (currently http://add-art.org/extension/image_set.xml)

* images_3.jar
This is an example of an art update. It contains all the banner art that is
being downloaded. The new chrome.manifest is set to expect that the art is
inside an "images" folder in the jar.

The filename & location of the jar update file is specified in the
image_set.xml file. The image_set.xml file also identifies when the
downloaded file "expires" and what set number it is associated with (used to
identify whether it is a new set).