option to use first bytes of stdout as message

Option -n transforms pn into a nagios plugin wrapper.
It passes the first line of output as message.

This also transforms nagios plugin conforming return values
into the proper monitoring events.
Rename test to match code to be tested.
adapt tests for real send_nsca
Use printf instead of echo -e (which is a bashism)
first 8KiB of stdout as message, not first line
Merge pull request #14 from Jimdo/nagios_plugin_wrapper

Add Nagios plugin wrapper