FsCheck 2.1.0

Switch to xunit 2.1. Note: Needs -noshadow argument on Mono due to Mono bug. A fix for Mono is submitted and is expected in Mono 4.3. (heroic achievement by Jon Hamm - thanks to all who tested early versions and found bugs.) xunit versions < 2.1 are no longer supported. The latest FsCheck version that supports xunit 1.9.2 is currently FsCheck.Xunit 2.0.7. Made Arbitrary overrides (e.g. using Arb.register, passing arbitrary to Config, or setting them in PropertyAttribute) thread-local. This is mainly to support xunit2: it runs tests in the same class in parallel by default. Not making the overrides thread local causes overrides from one test to mistakenly apply to others. FsCheck.Runner.init is Obsolete, it should not longer be necessary to call it explicitly. (though I am cautious here - please report any issues.) Fix for AppVeryor NuGet feed dependency problem.