flickr_photo plugin (fixes #3)

[jekyll-flickr](https://github.com/cnunciato/jekyll-flickr/) with a few
modifications:

 - Pull the Flickr API key from the `FLICKR_API_KEY` environment
variable
 - Class name changed from `FlickrPhotoTag` to `FlickrTag`
 - Tag name changed from `flickr_photo` to `flickr`
Kitten! (23 May 2011)