Fix for Command line options should be case insensitive #274

This fixes the second part of the issue. The option names can now be in any case.

I also changed the code for command line options to always lowercase the option when it is being created by the developer in case someone adds a non-lowercase option by mistake.

I just saw @CharliePoole's latest response. We can discuss if we want to only support case sensitive option names and drop this pull request if we do. We currently tend to use full names for most of our options, so I'm not sure we will ever have a use for different behavior for something like `-d` and `-D`. I don't have a strong opinion on this though.

If we decide to go with this, don't merge yet. I still need to make the change in NUnitLite.