Rewrite GenerateUID using a custom method

This custom method I wrote removes the dependency on System.Numerics and its BigInteger class, as suggested at <https://github.com/jasper-yeh/ClearCanvas/issues/22>.  I release this method to the public domain; the method can also be found at: <https://gist.github.com/peteroupc/c55d330d72c51d267eb4>.