Added the ability to generate media pages

- New optional parameter added to `Writer.__init__` to specify
the template file
- modified version of colorbox theme created that supports
generating media pages and linking to them (if javascript
is not supported)
- `media_page` plugin created that responds to the `gallery_build`
signal, and generates pages for each media element based on
the media.html template, including links to the next and
previous image (if applicable)