Create test runner packages during build

After upgrading Xunit to >= 2.3.0, package stopped being produced.

xunit/xunit#1520

Have to publish test runners to consume async changes

Reorganize in preparation for publishing new packages

Merge pull request #12 from kcamp/fix/GH-11

update build.bat to probe for version of msbuild compatible with C# 6

Version 3.0

* Standardize on targeting .NET 4.5.2 and NUnit 2.6.4

Start versioning at 1.0

Bump version for 2.1.2 release

Merge branch '2.1.1'

Merge branch '2.0'

Tweaks to publish script