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
Adapted check_http2 for use of new ShinkenPlugin class.
Clean: link.py->file_without_ext => link_without_ext->file.py

Better to have the file actually containing the python code be the one with .py extension.
And make the extension-less one links to the .py one.
bump check-http2 version.
Adapted tests for new ShinkenPlugin.