separate parsing and tools

1. Functions `tokenize` and `tree` are in the *sgfparsing.py* now.
2. Classes `Node`, `InvalidSgfException` and `CantProcessSgfException` are in the *sgfparsing.py* also.
3. Import logic has been changed from `from ... import ...` to simple `import`.
4. `uppers = set(string.ascii_uppercase)` has been moved from global scope to `tokenize` function.
5. Minor code formatting fixes.