Refactor PerfdataMetrics.split_value_and_uom()

It is now a module level function so it can be used
universally outside of perfdatametrics.

Conflicts:
	pynag/Plugins/relational_thresholds.py
	pynag/Utils/metrics.py
Utils.metrics: ensure python 2.6 compat
metrics.py - Improve docstrings and doctests
Merge pull request #204 from pynag/add-module-states-v3

Utils.metrics added functionality