Improve readability of complex favicons

Helps with [meta/9183](http://meta.discourse.org/t/favicon-notification-can-we-have-a-white-shadow-like-gmail/9183)

Improves the readability by adding a drop shadow behind the count and fading the favicon image a bit. This is important for people with really complicated favicons... like [this one](http://chrishunt.co/favcount/icons/il.ico).

When the count is removed or set to zero, the favicon is no longer dimmed.

before/after

![icons](https://f.cloud.github.com/assets/65323/987289/d18990de-08e8-11e3-9902-0f9883c01744.png)