Fix potential issue with NullTimer returning null context. Simplify null metrics.
upgrade xunit to v2
fix build script
bump version, push nuget
fix travis build
Merge branch 'master' into net40

Conflicts:
	Publishing/push-nuget.bat
	Src/Metrics.Tests/OwinAdapter/OwinMiddlewareTests.cs
	Src/Metrics.Tests/packages.config
bump version, push nuget