Create layouts, use base url and add example

Hey Dan.

I made some changes that you might want to look at so I didn't want to push directly and instead am giving it to you as a PR.

I created an examples page which uses one of the charts you created before (Plottable commits + members vs time.) The chart has been stylized slightly as well.

I added baseurl to _config.yml since it seems to be the best way to deal with the /plottable subdirectory, per: http://jekyllrb.com/docs/github-pages/

I'm still working on improving the home page.