Remove old solutions

This PR removes the old NUnitConsole and NUnitFramework solutions and does additional reorganization that was postponed while those solutions exist. Specifically...

1. NUnitFramework.sln and NUnitConsole.sln are removed
2. Build scripts have been moved to the root and tested, with the exception of the NAnt script for NUnitConsole. The NUnitConsole NAnt script has been deleted.
3. References to nunit.framework from the engine and console tests are now project references..
4. The engine and console tests now use net-2.0/mock-nunit-assembly and the extra mock-assembly project in the NUnitConsole directory has been removed.

Further removals / restructuring / improvements will be done after completion of issue #181