Serve topojson for every country at every admin level

[mikefab]

Task

Add the following end-points:

• shapefiles/countries - countries for which we have shapefiles
• shapefiles/countries/:country/:series - returns available shapefile series per country
• shapefiles/countries/:country/:series/admin_levels - returns available admin levels
• shapefiles/countries/:country/series/:series/admin_level/:admin_level - return topojson file

Why do this

Apps that visualize data from this API (ex: magicbox-maps, school-mapping) need to fetch shapefiles, and topojson is a efficient format to transmit.