Show long description on dataset page.

#### Description
Adds the ability to add an _optional_ `Long_Description` column to the Datasets spreadsheet. Any thing in that column will appear on the individual dataset page.

It renders html tags.

#### Screenshot
![screen shot 2014-05-30 at 11 50 01 am](https://cloud.githubusercontent.com/assets/595778/3134877/48a0ca58-e82b-11e3-97f7-3b117cff0d3a.png)