NUnit and NUnitLite Features Should be Kept in Synch

NUnitLite is intended to be a proper subset of NUnit but they periodically get out of synch. We need some way to ensure that any differences between the two are deliberate and that new framework features are automatically added to both programs by default.

To ensure this, NUnitLite will be built from the NUnit source using conditional compilation. NUnitLite builds will define NUNITLITE and the code will use it to determine what should be included or excluded.