externalize everyting into runner class

   improves tests and makes everything more expressive
move shim to resources
remove dependencies from Gemfile

dependencies should be specified in gemspec only
extract argument parsing into class
make passing of options available

pass the Gemfile name with -g or --gemfile
add an option for printing usage
move validations of arguments away from runner
extract method for input validation
add support for multiple gems
put logic for arguments validation into correct method