Added 404.md file and 404 layout file

initial custom 404 page.

feel free to change - I based the 404.html page off of page.html and removed some things like the 'share' button.

In the 404.md file you will notice that I had to put 404 in quotes - i.e. "404"; this is because without quotes using a layout name that begins with a number appears to be a bug w/ Jekyll. I thought having the 404 layout file named '404.html' the most straight forward approach.