- Rename 'default' theme to 'classic'.
- rename `themes/default` to `themes/classic`
- add new `themes/default` theme that derives from 'classic' theme for compatibility.
- setup.py depends to 'alabaster' and 'rtd' theme.
- theming.py import and run `alabaster.get_path()` to append path for alabaster theme.
- theming.py also import and run `sphinx_rtd_theme.get_html_theme_path()` to append path.
- sphinx-quickstart will generate `html_theme = 'alabaster'`.
- If user use 'default', sphinx emit notice (WARNING) suggestion to use 'alabaster' or use 'classic'.
- add documentation for new alabaster and readthedoc theme and link to original pages:
- https://pypi.python.org/pypi/alabaster
- https://pypi.python.org/pypi/sphinx_rtd_theme
