VS2015 Migration

[shiftkey]

Fixes #457 and does a bunch of other things:

• [x] Update to latest Fody package - cleanup old version
• [x] nuget.exe for tests updated as Fody package needs minimum version
• [x] AppVeyor integration enabled - use VS2015 image
• [x] address missing Microsoft.WebApplication.targets from old version of VS
• [x] why isn't the build greeeeeen?

[shiftkey]

I've added the UI tests to .\run-tests.cmd and they work locally, so I'm going to close this out once the PR is green again...