[css3-flexbox] added algorithms for pagination and flex distribution

I made two changes to the spec:

1)      Added algorithm for pagination. It has a couple of issues for dealing with overflow and dealing with forced breaks. There were good ideas at discussion with Fantasai, to be included next.

2)      Changed the algorithm for flex distribution. I am convinced that it will always finish and it does get the right result for min/max cases. Prove me wrong.