refs #1346. Add new default theme.

- 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

CHANGES

@@ -13,6 +13,14 @@ Incompatible changes
 Features added
 --------------
 
+* #1346: Add new default theme;
+
+  * Add 'alabaster' theme.
+  * Add 'sphinx_rtd_theme' theme.
+  * The 'default' html theme has been renamed to 'classic'. 'default' is still
+    available, however it will emit notice a recommendation that using new
+    'alabaster' theme.
+
 * Added ``highlight_options`` configuration value.
 * The ``language`` config value is now available in the HTML templates.
 * The ``env-updated`` event can now return a value, which is interpreted