Skip angle unit checks if value is 0

addWarning 'invalid-value' since unit identifiers are only optional for <length>s and expected for <angle>s (http://www.w3.org/TR/css3-values/#angle-value, https://developer.mozilla.org/en-US/docs/Web/CSS/angle), but don't throw an error.
Merge pull request #30 from crowjonah/patch-1

Skip angle unit checks if value is 0