Small tkinter-related fixes

* Use lower-cased tkinter module name (uppercased may fail in Python 3.x on case-sensitive filesystems, and lower-cased tkinter it is fixed by nltk.compat for Python 3.x);

* imports in nltk.sem.drt_glue_demo are fixed (I didn't try the demo, but the code looks OK - I think it should work in 3.x as-is)