added unit tests for bin/howitzer
refactored spec/bin/howitzer_spec.rb
refactored method #execute_generator
refactored #execute_generator with check if unknown and missing options specified
refactored #execute_generator with separate check for empty and unknown option
refactored bin/howitzer and fixed failing tests
Merge pull request #31 from alexspot/develop

added unit tests for bin/howitzer
Merge pull request #33 from trofymenko/develop

[TA] Cover /lib/howitzer/helpers.rb by tests