Shreds review page. Resolves #22.

 - Implemented a new page that contains current user's tagged shreds
 - Added a pagination macro
 - Moved nav to base.html with a block to extend
 - Next shred loading will only happen on the main page
 - Models now inherit flask-mongoengine's Document in order to obtain
   extended functionality
 - Small CSS fixes

#### Implementation Notes

I've implemented a paginated list although I have tried to do an infinite scroller first. However I've wasted too much time to get it working well with a scroller (via [Sly](http://darsa.in/sly/)) with either current markup or scripts (which are a bit of a mess anyway) getting in my way so I ended up with simple pagination. Probably worth revisiting a scroller implementation after #29 and some scripts refactioring.

Each shred item on the page contains an image (on the paper background available through `piece_in_context_fname` property) and a list of tags added by the current user.

Also I have used a `col-md-3` for each shred in the list so that on a non-mobile device it would fit 4 items in a row. It collapses to one item per row on `col-xs-*` and `col-sm-*`. Let me know if you have a better idea.