Eliminate Sphinx warnings

A bunch of cleanup to prevent Sphinx warnings:

##### Before:

```
$ rm -rf _build ; make html
sphinx-build -b html -d _build/doctrees   . _build/html
Making output directory...
Running Sphinx v1.2.3
loading pickled environment... not yet created
building [html]: targets for 9 source files that are out of date
updating environment: 9 added, 0 changed, 0 removed
reading sources... [100%] index
/Users/marca/dev/git-repos/retools/docs/api.rst:4: ERROR: Unknown interpreted text role "app".
/Users/marca/dev/git-repos/retools/docs/api/limiter.rst:4: WARNING: Title underline too short.

:mod:`retools.limiter`
--------------------
/Users/marca/dev/git-repos/retools/retools/queue.py:docstring of retools.queue:71: WARNING: Inline strong start-string without end-string.
/Users/marca/dev/git-repos/retools/docs/api/queue.rst:4: WARNING: duplicate label queue_module, other instance in /Users/marca/dev/git-repos/retools/docs/api/limiter.rst
looking for now-outdated files... none found
pickling environment... done
checking consistency... /Users/marca/dev/git-repos/retools/docs/api/limiter.rst:: WARNING: document isn't included in any toctree
/Users/marca/dev/git-repos/retools/docs/caching.rst:: WARNING: document isn't included in any toctree
done
preparing documents... done
writing output... [100%] index
writing additional files... (5 module code pages) _modules/index genindex py-modindex search
copying static files... WARNING: html_static_path entry u'/Users/marca/dev/git-repos/retools/docs/_static' does not exist
done
copying extra files... done
dumping search index... done
dumping object inventory... done
build succeeded, 7 warnings.

Build finished. The HTML pages are in _build/html.
```

##### After:

```
$ rm -rf _build ; make html
sphinx-build -b html -d _build/doctrees   . _build/html
Making output directory...
Running Sphinx v1.2.3
loading pickled environment... not yet created
building [html]: targets for 8 source files that are out of date
updating environment: 8 added, 0 changed, 0 removed
reading sources... [100%] index
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] index
writing additional files... (5 module code pages) _modules/index genindex py-modindex search
copying static files... done
copying extra files... done
dumping search index... done
dumping object inventory... done
build succeeded.

Build finished. The HTML pages are in _build/html.
```