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 doesn't make it clear what the relationship is (if any) between the `extent` option and the `constrainCenter` option.  

It would be clearer to say that the view's center is constrained if either the `constrainCenter` or `extent` option is provided (with the `constrainCenter` option taking precedence).

I know your change doesn't change the behavior of the `extent` option, but it would be nice to have its behavior better documented so people could understand when `constrainCenter` should be used instead.