Enable debug.
Changed the MathNodes HTML representation.

Now, a math node contains a span as RDFa property 'source'
containing the input formula.
Render MathJax errors in an appropriate order.
Minor fixes in ce.MathNode
dm.MathNode can create MML output when saving.

Along with the 'source' property element, there may
be a 'output' property element containing MathML output
rendered with MathJax.