[WO-373] Add callbacks for new/updated/deleted msgs

Listen to updates from browserbox
Keep internal cache for message uids/sequences
Introduce onSyncUpdate callback
Introduce selectMailbox method
Bump version and use browserbox v0.3.5
Added comments
Merge pull request #15 from whiteout-io/dev/WO-373

[WO-373] Add callbacks for new/updated/deleted msgs