Facebook signin

Hey @schofld,

I added the Facebook Signin HTML, CSS & JS.

I left it up to you to register you app via FB and copy/paste your appId and secret.

This is my first branch and pull request, so let's double check everything before we merge.

Just FYI, here was my workflow:
* I forked your master
* then created a meteor project on myside
* then cloned my fork into that meteor project
* then added the accounts-base package to remove an error/app crash
* then create a branch
* then added the HTML, CSS & JS
* then add, commit, push to my facebook-login branch
* then this pull request

Let me know your thoughts and if you need anything and how the test goes. Let's keep collaborating and cross off items on the todo list.