Scale images to fit the page for DOCX

Images that are bigger than the page were truncated. This will now
scale them if they are larger than the page. The scale is currently
hardcoded with Word "letter" defaults (page size and margins)
MediaBag: Improved normalisation when writing files
Merge pull request #1487 from mpickering/master

MediaBag: Improved normalisation when writing files
MediaWiki reader doesn't recognize german "Bild"
Merge pull request #1490 from ulikoehler/master

MediaWiki reader doesn't recognize german "Bild"
Docx reader: Use TeXMath to write math

The new version of TeXMath can translate from its type system into
LaTeX. So instead of writing the LaTeX ourself, we write to the TeXMath
`Exp` type, and let TeXMath do the rest.
Add a note on how `mapD` works.
Remove now-unnecessary TexChar

TeXMath does the work now.
Docx reader: remove now-unnecessary state variable.

This also introduces a `defaultDState` value.
Merge pull request #1491 from jkr/texmath-equations

Docx Reader: Use TeXMath for writing equations.
Get rid of unused docx variable.

Since changing the Docx type, this is no longer necessary. Thanks to
Matthew Pickering for picking up on this.
Merge pull request #1492 from jkr/nodocxenv

Get rid of unused docx variable.
pandoc:  Don't strip path off of sourceURL.

We need this information for relative URLs!

This should resolve the continuing problem noted in #750.
Docx Reader: Added Default instances and removed withDState

Signed-off-by: Jesse Rosenthal <jrosenthal@jhu.edu>
Docx reader: Split math out into math module.

Could use some cleanup, but this is the first step for getting
an OMML reader into TeXMath.
OMath module: Add new file.
Docx reader: Handle inline drawings.

Previous drawings that were under some other toplevel run (i.e., a
hyperlink) wouldn't be properly handled. This should fix that.
Docx reader: Test inline image code.
Merge pull request #1495 from jkr/inline-drawings

Docx reader: Inline image fix
Merge pull request #1494 from jkr/math-module

Math module