Master should display currently unassigned regions in UI

The master often knows that a region is unassigned, but thinks it's getting assigned. It would be very useful to display the current set of unassigned regions in the UI, as well as their current state (eg where it's supposed to be getting assigned, how long ago it last heard from it, etc). This would be useful in debugging/diagnosing cluster issues (eg I just noticed that a client had hung, took me a while to figure out that I had a region that was repeatedly failing to open with an NPE due to HBASE-2729)