Simplify C# build

This commit leverages Make to decide whether or not nunit-console.exe needs to be downloaded.
Get rid of shell script to run tests - make can do this too.
Only rebuild dlls when cs files have changed
Merge branch 'master' into simplify-csharp-build
How to build C# code
Merge pull request #1 from cucumber/simplify-csharp-build

Simplify C# build