Modal for Welcome text

fixes #25 

The modal is kind of plain though, could use a graphic or something.

I cheated, this is live. I'd still like to revise it, but wanted to at least get the plain modal out asap.

![screenshot from 2013-10-24 12 09 07](https://f.cloud.github.com/assets/64711/1398269/cbbbd67a-3c9c-11e3-9ea3-6d503ab58586.png)