Fix some login page issues.

- The login page has a fatal JS error ("no property or method pngFix") because `login/main.php` does not load the pngFix script. I've added a conditional to `global.js`. (The other option would be to add the script to the login page, but I prefer this way because it means I can now remove the pngFix requirement altogether for deployments that don't have to support oldIE).

- CSS tweak to fix misaligned divs:
Before
![before](https://f.cloud.github.com/assets/160227/817060/cd1a01ee-ef63-11e2-953d-c727bc14e23d.png)
After
![after](https://f.cloud.github.com/assets/160227/817062/d8d1ad84-ef63-11e2-8e1a-cce06dc8e3c1.png)