Verify that fast-shutter hardware triggering works

Tim Mooney recently re-implemented the hardware triggered fast shutter. I've added a ``enable_fast_shutter`` method to the TXM class, but it didn't work during 2018-03-10 beamtime. Tim came by afterwards and worked on it, but it now needs to be tested.

Also, we need to test that the rotary encoder can properly trigger the fast shutter during fly scans.