convert site to use Jekyll

I ported the site over to Jekyll so we don't have to continue hand-managing the home page. This will make it a lot easier to manage the page (and the team section specifically) as we grow.

A few notes:
* I've created a few includes to make the homepage code easier to follow. Right now, it's the slideshow, the main text, the exit link modal, and the contact form.
* For now, the blog is left as tumblr. It wouldn't be too hard to switch it over to using Jekyll for this, 
* the "use_github" variable in `_config.yml` can be easily flipped from 'false' to 'true' to bring the github links back. We could do something similar for linkedin or other links as needed
* the groundwork is here to have short bios for team members, though I have not implemented a template for this yet.
* The alphabetizing of the team section is a little bit hacky; I didn't want to use a plugin for this functionality in case we ever want to host this on github pages (which doesn't allow plugins). So team member files should be named `2013-12-31-team-LASTNAME.md` where LASTNAME is the last name.

A few notes:
* Deployment will use the `_site` folder instead of the root. Everything needed for the site is there, though.
* To contribute, jekyll will have to be running on the contributor's machine, or we'll need to run it on the server. I have no preference on that front.