*8825* Implement plugin gallery

Implement the plugin gallery.
Still needs steenking badges.
For now, the plugin list is fetched from http://pkp.sfu.ca/ojs/plugins.xml (where a sample file currently exists).
Actually, okay -- I re-read the comment here and you are assuming a potential version number after the plugin name.   Maybe we can make that more explicit, like ^([a-zA-Z0-9]+)(\-\d+(\.\d+)*)?$ or something, and then examine $matches[1]. Perhaps this isn't necessary.