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. 
This should read "the extent of France" rather than "the France extent."

Upon first read, I'm left confused by "a <code>change:resolution</code> listener may be used to use a constrained center when the resolution changes."  I'll see if reading the code makes things clearer.