improving speech-rule-engine integration

This fixes #49 and #50.

* the `result` will now have a separate `speakText` (not sure about the name)
* the MathML will now have the speech string in the alt-text (See inline comments -- I couldn't make the proper way work; for now a regex will have to do.)
Indeed, commas can make a difference, depending on the TTS and the screen reader. E.g., when I used the Festival engine for some of the Maxtract work, strategically placed commas made a huge difference. For ChromeVox, they would not. So for now there is no explicit punctuation. This is also in line with the MathSpeak rules at the moment.

Once we use SSML we should use proper serialised XML DOM anyway, which should solve the problem.