simplifies transparency check to only look for zeroes in all parameters....

... fixes overeager transparent keyword optimization

See additional comments in #257.
Yes, that is correct: the computed value of `transparent` is `rgba(0,0,0,0)`, [as per the spec](http://www.w3.org/TR/css3-color/#transparent-def), which is a different color than `rgba(255,255,255,0)`. It's the kind of thing one doesn't often come across, since transparent is transparent is transparent, but it **is** apparent when transitioning rgba values.

A transition from the `transparent` keyword (the equivalent of `rgba(0,0,0,0)`) to, let's say, `rgba(100,200,100,1)`, is a different effect than a transition from `rgba(100,200,100,0)` to `rgba(100,200,100,1)`. The former transitions both the color space and the opacity; the latter only transitions the opacity.