Fix Zeo timestamps and IDs

@beaugunderson, quick review? Getting these in for Sleepio, who's blocked by the
bad timestamps.

7a8bd9c (Kristján Pétursson, 2 minutes ago)
   Fix record IDs

   There's no native ID, so we generate one from the sleep record's bedtime.
   But, we were generating it from `Object.keys(bedTime)`, which is
   arbitrarily ordered, so for the same record, we could generate `6!`
   different IDs. Now we're consistent.

c51c354 (Kristján Pétursson, 46 minutes ago)
   Use sleep records' timestamp for `at`

   The default `at` is the time we performed the sync, which isn't helpful.
   Now we use the date Zeo reports on each sleep record.