Add a constrainCenter option to ol.View (WIP)

This is unfinished work. Probably needs discussions, tests and a better example. I'm just creating this PR for people to be able track progress on this. See [this post](https://groups.google.com/d/msg/ol3-dev/w7gLnpMr2nY/jRjqENSVhQUJ) and [this post](https://groups.google.com/d/msg/ol3-dev/QAdniHP-lsE/j_CHoswmhfQJ) on the mailing list for related discussions.