Add source and data download links

Implements and closes #129

Source and data download links have been added to the map page:

![screen shot 2014-07-09 at 5 02 17 pm](https://cloud.githubusercontent.com/assets/919583/3531888/b9c0f5d4-07b4-11e4-83ce-5441bdc572cb.png)

Source and data download on place detail pages:

![screen shot 2014-07-09 at 5 03 00 pm](https://cloud.githubusercontent.com/assets/919583/3531897/d292cd4e-07b4-11e4-866e-d3cbed612627.png)

All datasets have a source and a download link. The download link is in JSON format and based on the data you are currently viewing. 

For example, on the map page, it returns all data from one dataset for all of chicago: [http://chicago-atlas-staging.herokuapp.com/map/births_birth_rate.json](http://chicago-atlas-staging.herokuapp.com/map/births_birth_rate.json) 

On the place detail page, the download link shows just the data for that dataset in that neighborhood: [http://chicago-atlas-staging.herokuapp.com/place/near_west_side/births_prenatal_care_obtained_in_1st_trimester.json](http://chicago-atlas-staging.herokuapp.com/place/near_west_side/births_prenatal_care_obtained_in_1st_trimester.json)