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
EPUB Reader: Improved how images are extracted
EPUB tests: Updated test file
Org Writer: Write anchor elements

The Org Writer now writes empty span elements which have an id as an anchor.

For example `Span ("uid", [], []) []` becomes `<<uid>>`
Merge pull request #1496 from mpickering/master

Org Writer: Write anchor elements
Merge pull request #1488 from mpickering/epubfixes

EPUB Reader: Improved image extraction
HTML reader: Really ignore DOCTYPE and xml declarations.

This actually does what d71b013841f3c9c8c595591e312a31df16a728cb
said it did.

Revised epub tests to remove the repeated DOCTYPE and xml tags.
EPUB test renaming.

Renamed epub test files so they're identified more clearly as
epub:  features.{epub,native} -> epub.features.{epub,native},
and similarly with formatting.{epub,native}.

Added epub test files to cabal file, so they'll be included in
the tarball.
HTML EPUB exts: switch element can now be in either the inline or block position
OMath parser: don't group expressions if there's only one.
OMath: Start phasing out internal OMath type.

This is the first step in removing the intermediate OMath type, which we
no longer need since we're writing straight to TeXMath Exp.