[FOR REVIEW] Use SourceCodePro-Medium for all platforms.

This is a possible fix for #4391 

Use `SourceCodePro-Medium` on all platforms. Previously, we used medium on the Mac, and normal everywhere else.

Here is a Windows 8 screenshot with the "normal" weight:
![normal](https://f.cloud.github.com/assets/1060119/816216/351ebfc2-ef3d-11e2-8ca4-2c40d3aef2d3.PNG)

Here is a screenshot with the "medium" weight:
![medium](https://f.cloud.github.com/assets/1060119/816217/403a3d78-ef3d-11e2-8684-26cf2e49d6db.PNG)