Add an updateol make target

This commits adds an updateol make target. This target clones or pulls the master branch of the ol3 repo, builds ol.js, ol-simple.js ol-whitespace.js, and updates our copies of these files.

And it stops here. It doesn't `git add` the files, not does it commit or push anything. These are still manual steps. I don't think the Makefile should do git commands on the mf-geoadmin3 repo. That's our job, as developers.

Please review.