Added pypi version badge
Model.filter() now uses Utils.grep() internally

This was long overdue, filter() method now uses the newer faster grep()
internally.

As far as i can tell filter() still functions like it did before except that now
it supports more features.
Parsers.config.needs_reload() improvements

* Now works without parsing all object config files first
Improvements to pynag.Model.config

The module object Model.config was previously set to None
at import time, and then turned into Parsers.config object when
the need for parsing arises.

Now it is set to an unparsed Parsers.config
Parsers - Fix unit tests ...

... when nagios.cfg is not found and exception was raised