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.
Merge pull request #1494 from jkr/math-module

Math module