Icons should better identify transit mode and agency

[TransitScreen]

TNM should contain icons that distinguish stops for light rail, subway/urban heavy rail, commuter rail, bus, etc. at least when zoomed in.

In addition, TNM should support custom icons per agency, but fall back to a "standard" set when custom icons are not provided.

(Morphing this bug after discussion with Andy)

[TransitScreen]

Andy's implementation suggestion: "For TNM, the way to do this would probably be to either modify the DB to allow for user uploading of icon images through the admin console, linking those icons to the particular models for each agency, or giving each agency a custom CSS class that the user could then use to specify icons."

[TransitScreen]

DevSeed has a set of BSD-licensed point of interest icons in 10 and 15 px sizes at: http://support.mapbox.com/kb/mapbox-tiles/maki-poi-icon-set. They are working on a 10, 15, and 20 px set with a similar flavor, including lots of transit options.

[TransitScreen]

We have some icons now (not officially announced): https://github.com/mapbox/maki/

Including elevated rail AND subway!