improve format of dates in tideline navbar

Goals: (@skrugman please verify these design criteria)
- show one day in the navbar in one-day view, with the format 'Monday, January 1st' ideally but 'Monday, January 1' is OK for now if the '1st, 2nd, 3rd' stuff proves finicky (@nicolashery will be handling the formatting in blip now, FYI)
- show the endpoints in two-week view, with the format 'October 1st - October 14th' ideally but 'October 1 - October 14' is OK for now

NB: Date format issues will solve #25, and the first goal is #24.

Tasks:
- [x] one-day.js emits 'navigated' event as an array containing a single ISO 8601 date string
- [x] replace '12 am' tick labels on x-axis with 'Jan 1'-style dates
- [x] two-day.js emits 'navigated' event as an array containing two ISO 8601 date strings