MathMLinHTML: Fix deprecation warning

The use of `nodeValue` on `Attr` objects causes a deprecation warning in
Firefox since version 7.  This patch causes calls to `value` instead of
`nodeValue` for all `Attr` objects.

This closes #362.
Merge pull request #1257 from tarleb/mathmlinhtml-deprecation-warning

MathMLinHTML: Fix deprecation warning
change tabs to spaces in pandoc.cabal
EPUB writer:  include extension in epub ids.

This fixes a problem with duplicate extensions for fonts and
images with the same base name but different extensions.

Closes #1254.
RST reader: Remove duplicate 'http' in PEP links

The generated link to PEPs had a duplicate 'http://' in its URL.
Org reader: Recognize plain and angle links

This adds support for plain links (like http://zeitlens.com) and angle
links (like <http://moltkeplatz.de>).
Org reader: Read anchors as empty spans

Anchors (like <<this>>) are parsed as empty spans.
Render numbers in YAML metadata without decimals when possible.

The change to aeson > 0.7 caused numbers to be rendered with
decimals.  This change causes them to be rendered without decimals
wehn possible.
Require aeson >= 0.7.
Bumped upper bounds for http-conduit and network.
Markdown writer:  Use proper escapes to avoid unwanted lists.

Previously we used 0-width spaces, an ugly hack.

Closes #980.
Merge pull request #1260 from joehillen/tabs-in-cabalfile

change tabs to spaces in pandoc.cabal
EPUB writer: Fixed some idrefs to match changes in ids.
Org reader: Enable internal links

Internal links in Org are possible by using an anchor-name as the target
of a link:

[[some-anchor][This]] is an internal link.

It links <<some-anchor>> here.
LaTeX writer: Mark span contents with label if span has an ID

Prepend `\label{span-id}` to span contents iff `span-id` is defined.
Merge pull request #1265 from tarleb/org-links

Improvements handling of internal links
Bump version to 1.12.4.
Updated changelog.
HTML writer:  improved detection of image links.

Previously image links with queries were not recognized,
leading to use of an embed tag rather than an img tag.
LaTeX reader: Made `\nocite` work.

This adds nocite citations to a metadata field, `nocite`.
These will appear in the bibliography but not in the text
(unless you use a `$nocite$` variable in your template, of
course).