Give a modal explaining what the HMDA Explorer is

Inclusion of Vex library (thanks for the suggestion @theresaanna) for a simple modal interface.

The text here is completely off the cuff, so let me know what needs to change.

![screen shot 2014-08-11 at 6 01 20 pm](https://cloud.githubusercontent.com/assets/326918/3883600/581d16ce-21a3-11e4-9d14-f59d79f0acd7.png)