optimize CSS

* sort ruleset documents' order
* add same margin to `sidebar` and `mainContent`
* centering vertically GitHub icon and Feed icon
* add right margin to GitHub icon (same size as Feed icon)
* optimize table header padding
* decrease tag block size

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This doesn't change original design, just optimize.

Before:

![](https://raw.github.com/gist/929065/20130828200948.png)

After:

![](https://raw.github.com/gist/929065/20130828200803.png)