Move PluginHelper _perfdata from class to object

Was writing some unit tests, hit a weird issue:

```
>>> from pynag.Plugins import PluginHelper
>>> p = PluginHelper()
>>> p.add_metric('test', 10)
>>> print p.get_metric('test')
'test'=10;;;;
>>> p = PluginHelper()
>>> print p.get_metric('test')
'test'=10;;;;
```

I only moved _perfdata since that's what was in my way, the other "variables" might belong inside the object as well.