[css-transitions] Change the rules for automatic reversing of transitions to shorten the duration (and negative delay) based on the portion of the value space traversed instead of reversing and jumping into the middle of the timing function. Move the section on reversing of transitions to be a subsection of the section on starting of transitions, since it belongs there.

This implements the working group resolution resolved in
http://lists.w3.org/Archives/Public/www-style/2013Jul/0082.html
http://lists.w3.org/Archives/Public/www-style/2013Jul/0089.html
to switch to my preferred proposal for reversing of transitions.

Note that I'm leaving a link to the demo of options in a note in the
spec.

Author: dbaron