Vapor 3.1 branch created

We're trying out a system similar to what Swift lang does for Vapor 3.1. There's a branch named 3.1 now that will have master periodically merged into it until 3.1 seems ready for release.

As the PR notes, 3.1 does not have a decided release date yet. Any changes that add features or deprecate existing APIs should be made against this branch instead of master.