Modified to not attempt to run NUnit v3 tests

I also updated the version to 2.6.4 in case we release this. The only real changes are in TestAssemblyBuilder.cs where I check if the test assembly references nunit.framework or nunitlite with a version > 2.

There are large formatting changes because tabs got replaced with spaces.

I did not add unit tests because it would require too much setup and a copy of NUnit v3 tests. I tested by attempting to run both v2 and v3 tests and combinations of the both. All work fine, no tests are found in the v3 assemblies.

The only thing I don't like about this is that it silently drops v3 tests. The only way I know to surface the error is through an exception, but that stops the entire test run.

I am pretty sure that if we drop this in the current Visual Studio adapter, it will start correctly ignoring v3 tests. This might warrant another release of the adapter.
No need for the #if - we don't build V2 for either Silverlight or CF