Add build and link tests

This adds a continuous testing with [Proofer](https://github.com/gjtorikian/html-proofer). 

> Proofer is a set of tests to validate your HTML output. These tests check if your image references are legitimate, if they have alt tags, if your internal links are working, and so on.

You must [enable Travis](http://docs.travis-ci.com/user/getting-started/) to see it working.