Fixes: GH-1

Allows GitHub-style backtick syntax for code -- e.g.:

    ```python
    import os

    def add(a, b):
        return a + b
    ```

Allows markup like:

    Text with double__underscore__words.

And it will render the double underscores within the words instead of
taking them to mean to format as strong.

Solves PEP 8 line too long errors

Fixes the following test failure (e.g.:
https://travis-ci.org/pypa/readme_renderer/jobs/148390785):

```
        if "<" in expected:
>           assert out == expected
E           assert '<p>Here is s...nkey</a></p>\n' == '<p>Here is so...nkey</a></p>\n'
E             Skipping 734 identical leading characters in diff, use -v to show
E             - n class="s1">'Ruff!'</span><span class="p">)</span>
E             ?           -
E             + n class="s">'Ruff!'</span><span class="p">)</span>
E
E             - <span class="n">dog</span> <span class="o">=</span> <span class="n">Dog</span><span class="p">(</span><span class="s1">'Fido'</span><span class="p">)</span>
E             ?                                                                                                                     -
E             + <span class="n">dog</span> <span class="o">=</span> <span class="n">Dog</span><span class="p">(</span><span class="s">'Fido'</span><span class="p">)</span>
E               </pre>
E             Detailed information truncated (6 more lines), use "-vv" to show

tests/test_rst.py:31: AssertionError
```

I guess the output of [Markdown](https://pypi.python.org/pypi/Markdown)
changed a bit?

Also, the only case bool(markdown.markdown(?)) can give False looks
like when given an empty string, itself returning and empty string.

However, on very unparsable case, markdown.markdown() can raise a
ValueError, let's catch it so we can fallback later on txt rendering.

This allow to replace occurrences of:

    description_html = readme_renderer.rst.render(release["description"])
    if description_html is None:
        description_html = readme_renderer.txt.render(release["description"])
    release["description_html"] = description_html

to simply:

    release["description_html"] = readme_renderer.any.render(release["description"])

And gaining rendring or Markdown.