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)
Merged the colorbox media page theme into colorbox
Merge pull request #126 from jrstarke/f_media_pages

Added the ability to generate media pages