Update timestamps on DMs #30

The main timeline has a DeltaUpdater that updates the time delta on each tweet as time moves on. We don't currently appear to do that with DMs, so the DMs always show their delta compared to the first time you saw them.

This results in two behaviours if you keep the DM thread open:

• Every DM that was there at the start is still the same time in the past, even hours later
• Every single new DM will be at "now" (or up to 2min, because of the delay from DMing to polling and retrieving it)