sxross/MotionModel — pull request #128 by aviscasillas (2014-11-21 17:45:12+00:00)
Title: Validate with conditions

I just want to set starting point with conditional validations.
I think it'd be cool to provide some feature to execute validations just in some defined contexts.
In this PR I'm adding the `:if` option as a very silly way to provide this.