Started writing failed_logins and sudo_commands operations
Finished failed_logins and sudo_commands; need to fix log propagation
Corrected, polished new linux platform operations; fixed log propagation
Changed users operation to attempt 'sudo -n cat' before 'cat'
Corrected docstrings in credentials setter and to_xml methods
Merge pull request #17 from jeandesmorts/new_linux_platform__operations

Added new operations to Linux platform: failed_logins, sudo_logs