workaround for sphinx assertion error

We were hitting sphinx bug documented at
https://bitbucket.org/birkenfeld/sphinx/issue/1160

This workaround makes those assertion error dissapear and
sphinx to finish happily.
fix sphinx errors&warnings; some pep8
add docs/_build to gitignore
remove docs/_static folder.

It was giving troubles with debian dh_sphinxdoc, due
to an old, unrecognized version of jquery being there that
could not be symlinked to the js-sphinxdoc version.
Merge remote-tracking branch 'kalikaneko/bug/fix-sphinx-build' into develop
Revert "remove docs/_static folder."

This reverts commit e5351cfe957cfacab60dded18bc825aa5a4a685b.

As mentioned on the issue tracker, [0] some of the CSS from docs/_static was
still required to make the layout of the Sphinx generated output have the
correct layout. I reverted the last commit, which removed the whole
docs/_static directory, and then removed all the JavaScript and the unneeded
CSS. Hopefully, this is okay with Debian. :)

[0]: https://github.com/isislovecruft/python-gnupg/pull/27#issuecomment-29712108
Move DETAILS.html into _static/ so that the documentation can link to it.
Make doc html head font easier to read.
Add options to Sphinx conf.py for -Wall and rebuild environment.
Update Sphinx version requirement.
Add intersphinx mapping to docs.python.org.
Specify Sphinx source encoding explicitly.
Disable Sphinx prefixing with full module names.
Add reference Sphinx directives for module sections in docs.
Don't document special attributes in _meta twice.
Fix a couple refs pointing to the wrong classes in docs/gnupg.rst.
Fix blockquotes of previous documentation in gnupg.rst.
Remove duplicate headings from module docstrings.
Fix the rst table of logging level names and values.
Cleanup Sphinx directives in docstrings in _trust.py.