Pull in plugins automatically from github

[ewinslow]

This is a pretty effective search for finding elgg plugins on github

https://github.com/search?l=&p=2&q=elgg-plugin+extension%3Ajson&ref=advsearch&type=Code&utf8=%E2%9C%93

The idea is to minimize overhead for folks looking to publish Elgg plugins. They just put it up on github and we pull it in automatically. Tagged a new release? Automatically published on Elgg.org. Easy peasy.