move 0.4.6 whatsnew items to 0.5.0 whatsnew

We've merged a couple of API changes, so the next release should be 0.5.0 instead of 0.4.6. I'll move all of the items in the 0.4.6 whatsnew file to the new 0.5.0 whatsnew file.

I'd like to make the 0.5.0 release sometime next week. Many of the issues that are currently tagged with 0.5 will be changed to 0.6 or someday.