Shared: Added function insertMedia which is an alias for M.insert
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`
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.