Adds time estimate to MARK output in logger
Implements refractoriness in StimulusGroup

This is a first approach to refractoriness in StimulusGroup during
stimulus presentation. Due to implementation details this works only for
non-binary patterns so far.
Fixes bug in set_distribution(x) in StimulusGroup
Merge branch 'master' of https://github.com/fzenke/auryn