*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).
@asmecher, are you assuming that the file name will be a string of letters or numbers followed by .tar.gz only, or are you relaxing the pattern match a bit in order to accommodate a trailing version number here?  I ask because the pattern is only anchored at the beginning, not the end.