First working version.
Fixes living dates for Stephen, Thibault, and Pieter.
Adds EBMUD bill.
Converts using arrow to using plain datetime.dates.
Renames DateRange.end to .end_exclusive and fixes bugs associated with that.
Adds some integration tests.
Properly divvies up bills when people move in or out during their duration.
Changes min_people.
Renames ledger to split_bills.