Imgur history

This branch adds a history box to the side of the screen, replacing the current single link box. The links gradually fade out, to disappear entirely after five new images have appeared.

This branch also adds a couple of tweaks to try and make large images more manageable.

This pull request is currently incompatible with #370 (the history appears where the Reddit box would reside; easily fixed by shunting the box to be underneath the loading notification).

+ Added ``max-width`` to eliminate horizontal scrollbars
- Removed link box
+ Added history list
+ Added function to add to history list
+ Set visible history list cap to five
+ Added ``s`` hotkey to toggle ``overflow:hidden`` (and thus the scrollbar).