Add a config to set the footnote separator. Also remove rel/rev as they aren't really compatible with HTML5 and we already have classes set for refs and backrefs.

Fixes 723.

This was adapted from 11408e50 of the md3 branch.

Additional CSS classes names can be appended to the admonition name
using spaces as separators.

The following markdown:

    !!! note floatright
        This is a floating note.

Generates the following HTML code:

    <div class="admonition note floatright">
        <p class="admonition-title">Note</p>
        <p>This is a floating note.</p>
    </div>

All file-based tests are now defined as unittest test cases via a
metaclass which walks a directory and builds a unittest for each pair
of test files.

To run the tests just run `python -m unittest discover tests`. Or use
tox as the tox config has been updated to run the new tests and all nose
specific code has been removed.

The test generator tools have been removed as well. If any changes or
additions need to be made to tests, they should be implemented using
the new framework rather than with the file-based tests. Eventually,
only the PHP and pl tests should remain as file-based tests.

Preserve the line which a reference was on to prevent raw HTML indexing issue. Fixes #584.

Prevent raw HTML parsing issue in abbr and footnotes

Peserve abbreviation line when stripping and preserve a line for each footnote block.  Footnotes should also accumulate the extraneous padding.

Test extra lines at the end of references

Strip the gathered extraneous whitespace

When processing footnotes, we don't actually care to process the extra whitespace at the end of a footnote, but we want it to calculate lines to preserve.

Also consume to the end of the first line of any admonition. Everything after
the title is discarded. However, the only thing that will match should be
whitespace so it should be a non-issue.
Fixes #550.

Single column tables are valid tables, so add back in the accidentally
removed functionality of allowing single column tables, but with one
exception -- table bodies should not render empty (an empty 
`<tbody>` is invalid HTML. Fixes #539.

Fixes #412 and #493.  First we parse footnote content as
its own document avoid quirks with using li as a parent. Second, we
surround placeholders with STX and ETX to prevent them from interfering
with inline parsing; this is also consistent with how placeholders are
used everywhere else in Python Markdown.

Track when we find duplicate footnote references and create unique ids
for them.  Then add an additional tree-processor after inline to go
back and update the footnotes with additional back references that link
to the duplicate footnote references. Fixes #468.