Accessibility page (VPAT)

This pull request creates an `/accessibility` page describing GitHub.com, GitHub Enterprise, GitHub for Mac and GitHub for Window's compliance with §508 accessibility standards. The information in the pull request is to be considered an ongoing draft until merged.

The format is adopted from the industry-standard [voluntary product accessibility template (VPAT)](http://www.state.gov/m/irm/impact/126343.htm). There's some more background on [the itic site](http://www.itic.org/public-policy/accessibility):

> The Voluntary Product Accessibility Template®, or VPAT®, is a tool used to document a product's conformance with the accessibility standards under Section 508 of the Rehabilitation Act. The purpose of the VPAT is to assist Federal contracting officials and other buyers in making preliminary assessments regarding the availability of commercial "Electronic and Information Technology" products and services with features that support accessibility. 

[Firefox](http://www.mozilla.org/en-US/firefox/vpat-2.html) has a great example of a VPAT written for Humans that we can emulate in part.

## Rendered Preview

(Click for full screen view)

![vpat](https://f.cloud.github.com/assets/282759/2336387/f2609748-a48e-11e3-8245-6f2dffe37551.png)

(This pull request reopens https://github.com/github/government.github.com/pull/207 after a rogue merge.) 

/cc @github/accessibility