Removed taxonomy prefixing and added query variables

- playlist type prefixing removed due to changes to WP 4.1

- `media_in_gallery` and `media_in_playlist` query variables added.
Media in galleries and playlists can now be queries by entering the
gallery or playlist `ID` or `post_name`  into the new query variables.
The resulting `WP_Query` object will also have a 'gallery' or 'playlist'
property, which will hold information on said collection.

- Galleries now have date archive permalinks.