Merge pull request #50 from nunit/release-3.9.0

Release 3.9.0

Merge pull request #32 from nunit/release-3.8

Release 3.8 preparation

Merge pull request #18 from nunit/release-3.7

Prepare for 3.7 release

Update the build script to work on the release branch

Restructure build.cake to have similar layout to other extensions