Updates; convert to Python package

- setup.py and file renaming: convert to a Python package. The
  generated data files may be updated by running gb2260/__init__.py
  directly.
- README.md: reflow text.
- .gitignore: ignore setuptools files, python-jianfan (if placed in a
  subdirectory), others.
- extra.csv: information pertaining to the latest code table from the
  NBS website. This is provided to keep such information separate from
  gbt_2260-2007_sup.csv, which contains only information found in the
  actual GB/T 2260-2007 standard but not in gbt_2260-2007.csv. e.g.
  340181 is removed from gbt_2260-2007_sup.csv.
- init.py (renamed from gb2260.py) refreshed output in unified.csv