use open3 library for system calls
always calls zypper in a silent mode unless quite=false option is set
add ZypperError and SystemCallError
fix system tests
fix zypper tests
fix system tests
mute rubocop
100% code coverage
Merge pull request #107 from SUSE/review_140613__cnt_catch_zypper_command_line_errors_in_suseconnect

[Review] Request from 'vlewin' @ 'SUSE/connect/review_140613__cnt_catch_zypper_command_line_errors_in_suseconnect'