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.
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).
Update latex reader test for change in latex reader.

We use 'author' for metadata, not 'authors'.
COPYRIGHT: Specifiy the version of the GPL

All code in pandoc licensed under the GPL version 2 or later is just
marked as being licensed under the GPL.  There are multiple versions of
the GPL, most notably version 2, version 3 and the Affero GPL.  As to
avoid possible confusion, licensing info is updated to be more specific
about the GPL and its version.
COPYRIGHT: Remove outdated copyright statements

Some files are no longer distributed with pandoc, the respective
copyright and licensing information is removed.
README: Update variable defaults documentation

The documented default values of variables now agree with their true
default values as defined in Writer/HTML.hs.
A bit of refactoring that shouldn't change any semantics.

In preparation for a fix to #1267.
Fix #1267.

We now check the writerName for a lua script in pandoc.hs, so that
lowercasing and format parsing aren't done.  Note this behavior
change: getWriter in Text.Pandoc no longer returns a custom writer on
input "foo.lua".
Allow html4 as synonym of html as reader.

It already worked for writer.
Merge pull request #1268 from tarleb/copyright-housekeeping

Copyright housekeeping
ADded SmallCaps to Arbitrary instance.
Added Cite to Arbitrary instance.

See #1269.  This reveals some test failures.