Real sidebar calculations & polished drawing UI

A quiet Sunday night. In this pull:

- The sidebar’s cost and distance information is now real, and is updated live as you draw. These numbers still need to be refined (and probably set on a per-channel basis), but it’s good for demo-ing the concept. Resolves #5.
- Refined the line drawing / editing UI. There’s hints about how to draw, much clearer marker indicators, and tooltips for dragging markers. It also resets into ‘drawing’ mode when you delete the second-to-last marker — prevents strange situations when you just have a single marker. Resolves #2.
- Random route names for new transit lines. I was tired of seeing Untitled Line everywhere. Please add all your name suggestions to `app/assets/javascripts/data/names.js`.

I’m happy to start having code reviews these pull requests instead of auto-accepting if folks are interested and have time. Just speak up!