fix random strategy for question selection

issues: #27
check existence of default value

issues: #27
number of answers per A/B testing value

issues: #27
allow setting {max/min}_answers during A/B group initialization

issues: #27
improve command for manipulation with A/B testing

issues: #27
install requirements during deployment
better exception handling

issues: #27
load default values when A/B group is not valid for the given user

issues: #27