browser: add colors for WebKit browsers

Based off of the implementation from #67, except this one matches the Node.js colored output exactly, including the trailing `diff` value.

![screen shot 2014-05-30 at 10 43 27 pm](https://cloud.githubusercontent.com/assets/71256/3139768/b98c5fd8-e8ef-11e3-862a-f7253b6f47c6.png)

From here I want to abstract the common "colorizing" logic into the common `debug.js` file, to DRY up the node and browser files a bit more.

/cc @Buritica @netwjx @lvivier @zackbloom