Fix broken links and link-formatting in CONTRIBUTING.md

Many of the links in CONTRIBUTING.md where broken because of bad link-formatting or mis-spellings.

Before:

![screen shot 2014-04-27 at 5 45 05 pm](https://cloud.githubusercontent.com/assets/4745181/2812775/60640492-ce6e-11e3-8457-e7f20267b69f.png)

After:

![screen shot 2014-04-27 at 5 45 41 pm](https://cloud.githubusercontent.com/assets/4745181/2812778/73f734ac-ce6e-11e3-99cb-9694b91cb2cb.png)

This PR fixes most of these issues. There is one link on line 95 that I just don't know where to direct to because there's nothing similar to it in the links on the bottom of the document.