Add selection mixin documentation

Completed in commit a406763b9693dbe1503c9ac3cd6970e8767b1f76.
Update selection documentation for simplification
Update complete-list and mixins pages for selection mixin
Merge pull request #545 from Bionicrm/gh-pages

Add selection mixin documentation
Add Selection mixin to navigation
Fix selection mixin code spacing
Merge pull request #549 from Bionicrm/gh-pages

Fix selection mixin code spacing
Add updated Syntax for Analytics

As of Jan 2013 Google updated the analytics code.
http://analytics.blogspot.com/2013/10/an-easy-way-to-upgrade-to-universal_23.html
Update footer Twitter links

* Fixes #552
Refine docs for border radius shorthand mixin
Refine documentation for transition mixin

- Fixes #209
Adds example for mutliple transforms

- Fixes #493
Updates clearfix documentation
Refine documentation for the modular scale function