Heh. It was double-wrapping set operation results :)
Allow external operator use

Conflicts:
	src/Ruler/Variable.php
	tests/Ruler/Test/VariableTest.php
Register external operators on RuleBuilder instance
Update README section on external Operators.
Refactor code duplication in wrapping Operators with Variables.
Registering non-string namespace should throw InvalidArgumentException.
Merge pull request #30 from bobthecow/feature/external-operators

Allow external operator use