Nudge to correct alignment of file and status icons

I noticed you increased the size of the file icon @kaepora and I think it looks good. Nice move!

This is a small change to correct the alignment and positioning of the status icons that overlay it. I also switched to using an octicon glyph to render the file icon instead of using a PNG.

In current master it looks like this (see how the download arrow is not quite centered on the icon):

![download icon before](https://cloud.githubusercontent.com/assets/8218578/3717152/47846dea-1621-11e4-9135-6ec3e4a3eaa7.png)

With these changes it looks like this:

![download icon after](https://cloud.githubusercontent.com/assets/8218578/3717159/6e329ade-1621-11e4-9995-65e08de2b9c8.png)