Added NUnit 2.6.3 test runner.

nunit-console added async/await in version 2.6.3. Added the
runners nuget so that this version can be used to
execute the tests correctly.

To run:
`mono packages/NUnit.Runners.2.6.3/tools/nunit-console.exe -labels -noshadow ~/Source/Force.com-Toolkit-for-NET/src/ForceToolkitForNet.FunctionalTests/bin/Debug/Salesforce.Force.FunctionalTests.dll ~/Source/Force.com-Toolkit-for-NET/src/ForceToolkitForNet.UnitTests/bin/Debug/Salesforce.Force.UnitTests.dll ~/Source/Force.com-Toolkit-for-NET/src/CommonLibrariesForNET.UnitTests/bin/Debug/Salesforce.Common.UnitTests.dll ~/Source/Force.com-Toolkit-for-NET/src/CommonLibrariesForNET.FunctionalTests/bin/Debug/Salesforce.Common.FunctionalTests.dll`