Reorder .gov domain dir/URL structure, and rewrite README

![blogdotgov_59681092small_2_0](https://cloud.githubusercontent.com/assets/4592/5484559/1f72faee-8659-11e4-86e2-ba3cfa769769.jpg)

This simplifies the directory and URL structure, and limits the focus to the full list. We can always update the README to link to the broken out datasets if people find them useful, but let's see what happens.

* Example new download link: https://gsa.github.io/data/dotgov-domains/2014-12-01-full.csv
* Example new viewer link: https://github.com/GSA/data/blob/master/dotgov-domains/2014-12-01-full.csv