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.)
I figured, but wasn't sure.  It may be that in the long run we will end up with strings with additional information in them for things like pauses, pitch changes, and so on.  I think that SSML uses internal tags to add that information, so I just wanted to point out the potential future issue.

One thing I have been thinking about is whether using commas to add pauses would be helpful.  I did this in the prototype that I did several years ago and I think it made a big difference.