Create Cite element even if no matching reference in the biblio.

* Add ??? as fallback text for non-resolved citations.
* Biblio:  Put references (including a header at the end of
  the document, if one exists) inside a Div with class "references".
  This gives some control over styling of references, and allows
  scripts to manipulate them.
* Markdown writer:  Print markdown citation codes, and disable
  printing of references, if `citations` extension is enabled.

NOTE:  It would be good to improve what citeproc-hs does for
a nonexistent key.
Merge branch 'altcite'
adding some cedilla characters to the LaTeX reader
cedilla-o breaks the compile, removing again
Merge pull request #960 from semorrison/master

Processing some additional cedilla accents while reading LaTeX
LaTeX reader: Added o-cedilla.
Write id for code block to label attr in latex when listing is used

The code:

    ~~~{#test}
    asdf
    ~~~

gets compiled to html:

    <pre id="test">
    asdf
    </pre>

So it is possible to link to the identifier `test`

But this doesn't happen on latex

When using the listings package (`--listings`) it is possible to set the
identifier using the `label=test` property:

    \begin{lstlisting}[label=id]
    hi
    \end{lstlisting}

And this is exactly what this patch is doing.

Modified LaTeX Reader/Writer and added tests for this.
Improved error reporting on filters.

Avoid showing spurious output and avoid double error messages.
Moved most of Text.Pandoc.Readers.TeXMath to texmath 0.6.4.
Removed dependency on citeproc-hs.

Going forward we'll use pandoc-citeproc, as an external filter.

The `--bibliography`, `--csl`, and `--citation-abbreviation` fields
have been removed.  Instead one must include `bibliography`, `csl`,
or `csl-abbrevs` fields in the document's YAML metadata.  The filter
can then be used as follows:

    pandoc --filter pandoc-citeproc

The `Text.Pandoc.Biblio` module has been removed.  Henceforth,
`Text.CSL.Pandoc` from pandoc-citations can be used by library users.

The Markdown and LaTeX readers now longer format bibliographies and
citations.  That must be done using `processCites` or `processCites'`
from Text.CSL.Pandoc.

All bibliography-related fields have been removed from `ReaderOptions`
and `WriterOptions`: `writerBiblioFiles`, `readerReferences`,
`readerCitationStyle`.

API change.
Updated travis build.
Removed citeproc-hs from osx, windows build process.
Removed tests relating to citation processing.
Added `-F` as shortcut for `--filter`.
Merge pull request #961 from nougad/add_latex_listings_label

Write id for code block to label attr in latex when listing is used
Added markdown citation parsing test.
Merge branch 'master' of github.com:jgm/pandoc
LaTeX reader: Allow accents with combining characters.

accent now returns [Char], not Char.
Generalized type of stringify.