[css-speech] voice-rate "computed value"

I believe that "specified value" for voice-rate was incorrect (because all of [normal, x-slow, slow, medium, fast and x-fast] depend on the active voice, so the computed value must "carry" the keyword and optionally the percentage). This is essentially the same problem we had with pitch and range.
I fixed the issue, and to make the property consistent with voice-volume and voice-pitch, I also allowed combining a keyword with a percentage. A new example is provided to illustrate the mechanism:

http://dev.w3.org/csswg/css3-speech/#voice-rate