Attempt to fix issue #858

Hello

Here is an attempt to fix issue #858. It works with perfdata containing spaces and optionally surrounded by single quotes.

It does not handle range format for warn and crit.

Tested on our production system with Shinken sending perfdata to Graphite. Maybe the same problem exists with PNP4Nagios, but since we don't use it, I didn't have the opportunity to check it.