Add a constrainCenter option to ol.View

IThis 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.

EDIT: and [this question](http://stackoverflow.com/q/27200086/1064422) on StackOverflow. 
Not necessarily your responsibility here, but it would be nice to make it easy to have this work with rotated maps as well.