EPUB writer:  Download webtex images and include as data URLs.

This allows you to use `--webtex` in creating EPUBs.
EPUB writer:  math with `--webtex` is automatically made self-contained.

And some code cleanup.
Shared.openURL:  Properly handle data: URIs.
Set default stack size to 16M.

This is needed for some large conversions, esp. if pandoc
is compiled with 64-bit ghc.
Added CPP directives to avoid warnings.

For 'import Prelude hiding (catch)'.  catch is no longer in
Prelude starting with base 4.6.
Add --atx-headers support to asciidoc and dont print empty identifier blocks ([[]]) on headers
Merge pull request #868 from maxandersen/atxasciidoc

Add support for --atx-headers in asciidoc and remove unnecessary [[]] generation for headers.
README: Note that `--atx-headers` affects asciidoc as well as markdown.
LaTeX writer: Always create labels for sections.

Previously the labels were only created when there were links to
the section in the document.

Closes #871.
Latest template changes:  title attribute on title section in dzslides.
Textile reader:  Correctly handle entities.
Markdown writer: Don't wrap attributes in fenced code blocks.
RST reader:  don't insert paragraphs where docutils doesn't.

rst2html doesn't add `<p>` tags to list items (even when they are
separated by blank lines) unless there are multiple paragraphs in the
list.  This commit changes the RST reader to conform more closely to
what docutils does.

Closes #880.
Markdown reader: fixed bug in unmatched reference links.

The input

    [*infile*] [*outfile*]

was getting improperly parsed:  "infile" was emphasized, but
"*outfile*" was literal.  This was due to failure to parse the
"fallback" in the reference link parser.

Closes #883.
Added a test for #833.
Improved code readability.
Properly handle blank line at beginning of input.

Closes #882.
Markdown reader: Check for blank lines first in blocks.

(And skip them.  This might speed things up in some cases.)
Added test for #882.
Documented `citecolor` variable