mix IDE M2

Integrated debugger;
Hit F5 and you can step forwards/backwards through execution.

prettyU256 in AlethZero return HTML content. 
prettyU256 in CommonJS return PlainText.
I thought it was not necessary to move prettyU256 (the HTML one) from AlethZero to CommonJS because this function might be removed in the future (I think we don't want to make rendering directly in the C++ side).