Org reader: Better module description, minor style changes

Use module description analogous to the markdown reader's.
Use (<$) where it makes sense.
Org reader: Fix code for subexpression parsing
Org reader: introduce Reader environment around Blocks/Inlines

This introduces a Reader environment in the style of
Text.Pandoc.Parsing.F, but adapted to the Org reader parser.
Org reader: Support footnotes
Org reader: Support more types of '#+BEGIN_<type>' blocks

Support for standard org-blocks is improved.  The parser now handles
"HTML", "LATEX", "ASCII", "EXAMPLE", "QUOTE" and "VERSE" blocks in a
sensible fashion.
Org reader: Fix parsing of loose lists

Loose lists (i.e. lists with blankline separated items), were parsed as
multiple lists, each containing a single item.  This patch fixes this
issue.
Org reader: Add support for plain LaTeX fragments

This adds support for LaTeX fragments like the following:

```
\begin{equation}
\int fg \mathrm{d}x
\end{equation}
```
Org reader: Apply captions to code blocks and tables

The `Table` blocktype already takes the caption as an argument, while code
blocks are wrapped in a `Div` block together with a labelling `Span`.
Org reader: Fix distinction of images and normal links

Fixed a false assumption about the precedence of (&&) vs (||).
Org reader: Fix parsing of footnotes

Footnotes can consist of multiple blocks and end only at a header or at
the beginning of another footnote.  This fixes the previous behavior,
which restricted notes to a single paragraph.
Move `compactify'DL` from Markdown reader into Shared

The function `compactify'DL`, used to change the final definition item of a
definition list into a `Plain` iff all other items are `Plain`s as well, is
useful in many parsers and hence moved into Text.Pandoc.Shared.
Org reader: Allow for compact definition lists

Use `Text.Pandoc.Shared.compactify'DL` to allow for compact definition
lists.
Add license statement for org reader files

This adds a brief note to the COPYRIGHT file, declaring the code in the
org-reader files to be licensed under the GPL.
Merge pull request #1256 from tarleb/org-reader-improvements

Org reader improvements
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.