Merge pull request #42 from vbrown608/dollars-by-zip

Fix cross-chart axis labels
Add hacky way to deal with Socrata data

This pulls the Socrata data every 24 hours and caches it in memory. But
only in the `production` RACK_ENV - otherwise the data_local.csv is
used, which actually has a slightly different format than the Socrata
data (hence the conditional in app.js). This will be rectified with a
future commit.
Only load from Socrata the contributions we care about
Changed format of map files from geo-json to topo-json to reduce space
Top bar of donor zipcode map legend darkens on mouseover.