\MathQuillMathField{}'s listed in .innerFields

Static math instances now have a .innerFields list of the
\MathQuillMathField{}'s that are inside.

How: whenever the LaTeX is set (including on initialization), do a
post-order traversal running .registerInnerField() with an array to
register inner fields on.
Add optional arg to \MathQuillMathField[name]{contents}

See test case
Move .initExtractContents and .latex into .initRoot #simplify
Refactor .init{Root,Events} as .initRootAndEvents

...on MathQuill.EditableField
Inline .initExtractContents into .initRoot #refactor
Rearrange AbstractMathQuill::initRoot #reorg
Merge branch 'dev' into feature.innerFields

Conflicts:
	src/publicapi.js
	- feature.innerFields refactored MathQuill.{Math,Text}Field in
	  src/publicapi.js
	- #319 on dev moved MathQuill.{Math,Text}Field from publicapi.js
	  to src/commands/{math,text}.js (respectively)
Merge pull request #321 from mathquill/feature.innerFields

Feature: .innerFields on static math instances