FIX: Creating styles for admin notice popup (Growl style)

When a page/object is modified/created in the CMS admin, a black popup appears in the top right. Currently this incorporates crude built-in styles. I have prettied this popup to create a more visually appealing style.

See screenshot below of before vs after:

![silverstripe_framework_fix](https://f.cloud.github.com/assets/1711444/858601/c5065e00-f56a-11e2-9ab3-0770785cf1a1.png)