MWMath cleanup

VisualEditor.php
* Add CSS file

ve.ce.MWMathNode.js
* Wrap the image in a span, so GenerateContentNode doesn't
  try to nest an image inside an image
* Remove unnecessary attribute setting
* Only pass unwrapped image to deferred.resolve
* Retrigger MathJax rendering

ve.ce.Node.css
* Use inline-block for image wrapper

ve.dm.MWMathNode.js
* Mixin GeneratedContentNode and implement getHash
* Copy over functionality of MWTransclusionNode:
  + Just store data-mw for attributes
  + Store orignal(DomElement|MW|Index) for selser

ve.init.mw.ViewPageTarget.js
* Add mwMath to the toolbar

ve.ui.MWMathInspector.js
* Remove static.InputWidget, not required in this architecture
* Use multiline TextInputWidget
* Only update mw attribute
* Allow creation of new math nodes

ve.ui.MWInspector.css
* Set height of TextInputWidget

Change-Id: I520f8ccc9f89a2ce70aa1d9e02ed0c6cacbecc2f