Updates to get income data to work

[jgradim]

• Update shapefile for census tracts, use all DC area shapefile
• Update blue color ramp to not use black as the last color stop
• Use "urban yellow" for hovers
• Provide a full transparent white as default for background for choropleth areas without data
• Change choropleth opacities
• Blacklist geo2010_nf, indc columns in CSVs
• Use relative paths on imports / requests to ensure app works on nested paths
• Update README.md with clearer instructions on how to configure dropdowns
• Host Lato font locally
• Force downcasing of all destination paths when converting / copying data files