Refactor RuleBuilder variable, property classes.

Separate the RuleBuilder fluent DSL concerns from the base Variable and VariableProperty classes.

The RuleBuilder VariableProperty would have been better implemented as a trait, but PHP 5.3, eh?
Add a VariableProperty trait.

Use this when creating extended RuleBuilder DSLs with custom operators and magick like that.

It doesn't do *us* any good, since we're targeting PHP 5.3 with this library, but it does help on the extensibility front.
Merge pull request #17 from bobthecow/feature/builder-concerns

Extract the RuleBuilder DSL from the base Variable and VariableProperty classes.