[Bug 825538] [email/activesync] Need to allocate unique device ID's for multi-device access to ActiveSync.

Track observed device ids and be able to report them.

Some apparent sanity-checking logic I added in the past but never got around to
releasing.
Merge pull request #16 from asutherland/device-id

[Bug 825538] [email/activesync] Need to allocate unique device ID's for multi-device access to ActiveSync. r=squib