Fix: PerfData: set values to '' only if None.
Merge pull request #20 from savoirfairelinux/fix_null_perf_data

Fix: PerfData: set values to '' only if None.
Merge pull request #19 from savoirfairelinux/new_plugin

New plugin
Enh: check_linux_bandwidth: use new ShinkenPlugin.
add unittest.
tests: Fix  help & version tests.