GeezerVison

* :bug:`73` Clean up some problematic font issues:

- Remove the outright broken Goudy Old Style, plus other mostly Adobe-only
 fonts, from the ``font_family`` config setting; it is now simply ``Georgia,
 serif`` which is what the majority of users were rendering anyways.
- Clear out the default value of ``head_font_family`` (which contained
 ``Garamond``, a nice but also Adobe only font)
- Set ``head_font_family`` so it falls back to the value of ``font_family``
 unless a user has explicitly set it themselves.

.. note::
 You can always go back to the old values by :ref:`explicitly setting
 <theme-options>` ``font_family`` and/or ``head_font_family`` in your
 ``conf.py``'s ``html_theme_options``, e.g.::

     html_theme_options = {
         'description': 'My awesome project',
         'font_family': "goudy old style, minion pro, bell mt, Georgia, Hiragino Mincho Pro, serif",
     }

.. warning::
 Depending on individual viewers' systems, this change *may* be **visually**
 backwards incompatible if you were not already overriding the font
 settings and those users had the fonts in question (which are not default
 on most systems).

 As seen in the note above, you can **always** override the new defaults to
 go back to the old behavior, using your config file.

* :feature:`18 backported` (via :issue:`101`) Add optional *next* and
*previous* links at the top and bottom of page content. Use theme option
``show_relbars`` to enable these. Credit: William Minchin.
* :support:`- backported` Miscellaneous project maintenance updates such as
adding to Travis CI and enforcing the use of ``flake8``.
* :feature:`110 backported` Add ``badge_branch`` option allowing
configurability of which specific Git branch the Travis, Codecov, etc buttons
default to. Credit: ``TitanSnow``.
* :support:`111 backported` Add setuptools-level entrypoint for improved theme
distribution compatibility. Thanks to Aaron Carlisle for the patch.