Resizable but static width panel

Some of my collaborators have long last names which makes a dynamic width
annoying. Because of this (and another reason, see below) I decided to
implement a static-width panel that's resizable.

This pull request adds a resize-handle and some style tweaks to make things look good
at varying widths, including fixed-width dates (with configurable format strings).

![image](http://recordit.co/UzlhdRxAl3.gif)

(For some reason the `cursor: col-resize` doesn't show up in the gif)

The other reason a static-width panel is better is because when blaming large
files in repositories with lots of history there is a noticeable delay before
the panel appears. It would feel much better if it displayed instantly, but
this is problematic in the current implementation because the width is unknown
until blame returns.

With a static panel width, the panel can react immediately to the keypress and
display a throbber to indicate blame is running. Then when blame completes, the
panel can update without resizing.

I haven't actually implemented instant display yet but I intend to.
My intent isn't to differentiate, it's to save horizontal space. I'm trying to save space because in this change I added 2-4 characters to the date string. No matter what I set the initial width to, some names are going to be cut off. So I'm trying to show as much of name as possible.

The hash doesn't need to be big because the actual string is pretty meaningless. I'd be happy with chopping off a few characters from it actually. 

I don't like 95% because that's only a one pixel difference for me at a font size of 15px. Compare 80 and 100 - do you still think it's too small?

100%
![image](https://cloud.githubusercontent.com/assets/4427/3580196/adde9c84-0bcf-11e4-847f-d65a988253c6.png)

80%
![image](https://cloud.githubusercontent.com/assets/4427/3580198/bb7663ea-0bcf-11e4-88f4-bbf13accbf34.png)

If I haven't convinced you I guess I can increase the font and move this to my user stylesheet ;)