Remove colored exception output

Trying to import IPython exception printer imports all kinds of things,
resulting in slow concert binary run-time without doing anything. If this is
really required it should be moved to a function that imports the formatter at
run-time. However, I strongly object this because the async module should be
free of any view related code.
Make dark_row and flat_row keyword arguments

in FlatCorrectedBackproject. One might want to set it just before
the reconstruction starts.
Make the default color blue even after clear

if it is not set by the user explicitly in PyplotViewer.
Fix colormap limits for images with zero variance
Simplify compute_rotation_axis

just subtract the mean, hence we don't need to care
about image semantics.
Add async.resolve() to get result of future lists
Change scan to yield futures instead of one list
Merge pull request #265 from ufo-kit/async-scan

Proposal: asynchronous execution of scans
Add writable property to ParameterValue
Test writable property
Enable stash/restore only for writable params
Test saving for Parameter
Merge pull request #267 from ufo-kit/fix-param-stashing

Fix param stashing for whole devices
Fix None handling of minimum and maximum in scan
Update scan in test_measures

fixes #265
Make State a subclass of Parameter

and use the hardware state over the software one
if possible.
Split transition and state checking
Refine FSM errors
Update tests
Fix state getter on dummy ContinuousRotationMotor