add result-per-story command to group by story the hours spent on the sprint
compare result-per-story to estimations and print percent of achievement
Merge remote-tracking branch 'origin/dev' into hours_per_story

Conflicts:
	lst/__init__.py
	lst/commands.py
first attempt at outputing a graph for stories result
improve graph rendering
render the axis above the series
take all stories into account (even the ones without any pushed hours) and some code improvement
update readme
Merge remote-tracking branch 'origin/hours_per_story' into dev
update readme