display stable npm version and installation instructions in footer

This is an implementation of https://github.com/npm/docs.npmjs.com/issues/10, made possible by yesterday's [updates to semver.io](https://github.com/heroku/semver.io/pull/17). Here's what it looks like:

![screen shot 2014-12-09 at 10 16 46 am](https://cloud.githubusercontent.com/assets/2289/5362972/c36900b8-7f8c-11e4-8240-61cf3c9ee757.png)

// @hunterloftis