Tumbleweed support

Beside other, small updates, this release includes first support for the rolling release named openSUSE Tumbleweed.

Per default, a Tumbleweed installation which is older than 30 days will trigger a warning state - and an installation older than 60 days will trigger a critical state. You can fine tune this behavior with the new command line options --tw_outdated_warn and --tw_outdated_crit.

Other changes:

• use proper Copyright
• use Perl modules: POSIX and Time::Local
• die, if release-file could not be opened
• get rid of get_distribution_from_os_release() function: use get_distribution() for all cases
• new test_tumbleweed() and check_returncode() functions (internal)
• trim quotation marks in trim() function
• added some os-release files for testing