Add and export View2D.calculateExtent

This PR suggests to rename View2D:getExtent to View2D:calculateExtent, as discussed in this ol3-dev thread: <https://groups.google.com/d/msg/ol3-dev/kJntPZCcsQ0/QlZFafa-GHEJ>. It also exports calculateExtent for external use, which was @adube's initial request.