README: turn all extensions into headers for direct linking

Change all `**Extension: ...**` lines into headers to allow permalinking to a specific extension.
README: reduce 1:1 Topic -> Extension nesting

Reorganize some sections whose only purpose was containing a single Extension subsection.
Added note in README about pipe tables not wrapping in LaTeX/PDF.

See
http://stackoverflow.com/questions/25037357/pandoc-long-tablerows-in-markdown-pdf-documents-do-not-get-linewrap/25050474#25050474
Merge pull request #1472 from mpickering/master

Shared: Added function insertMedia which is an alias for M.insert
Merge branch 'master' of github.com:jgm/pandoc
Made MediaBag a newtype, and added mime type information to media.

Shared now exports functions for interacting with a MediaBag:

- `emptyMediaBag`
- `lookuMedia`
- `insertMedia`
- `mediaDirectory`
- `extractMediaBag`
New module, Text.Pandoc.MediaBag.

Moved `MediaBag` definition and functions from Shared:
`lookupMedia`, `mediaDirectory`, `insertMedia`, `extractMediaBag`.
Removed `emptyMediaBag`; use `mempty` instead, since `MediaBag`
is a Monoid.
Docx tests: rewrite mediabag tests.

This will allow us to test the whole mediabag (making sure, for example,
that images are added with the correct keys) instead of just individual
extracted images. We compare each entry in the media bag to an image
extracted on the fly from the docx. As a result, we only need one file
to test with.

The image in the current tests was also replaced with a smaller one.
Remove now unneeded JSON test file.
pandoc.hs:  Rewrote some of the logic for clarity.
README: blank line after headers
EPUB Reader: Added EPUB reader
Except Compat: Updated to export more module functions
Options: Added option to turn on epub html extensions
HTML Reader: Extended HTML Reader to recognise EPUB specific elements
HTML Reader: Added support for anchors on links and list items
HTML Reader: Added ability to read MathML formatted <math> blocks
EPUB Reader: Integrated into program
EPUB Reader: Added tests
README: Updated with EPUB information