Extract Facter execution methods to mixin

The command execution methods in Facter::Util::Resolution were
cluttering up the class and breaking the single responsbility principle.
Leaving them around increases the complexity of refactoring the
Resolution class.

This commit extracts methods related to command execution to a mixin and
extends the Resolution class with the mixin to maintain compatibility.
In the long term we should deprecate the compatibility methods.
Move execution module from util to core
Merge pull request #590 from adrienthebo/facter-2-extract-execution-methods

Extract Facter execution methods to mixin
Merge branch 'facter-2'

Conflicts:
	lib/facter/util/resolution.rb
	spec/unit/util/resolution_spec.rb