Some charter changes

Based on the meeting today, I have made a few wording changes to the
charter. In particular...

1. The intro text was modified to focus on the three goals identified for
MathML needs to be useful for: presentation, accessibility, and
searchability. The later point probably needs more text describing what is
meant by "searchibility". Note that computability is *not* on that list.

2. That success means authoring tools actually generate and consume
semantics. I expanded the existing bullet point about 'using core' in the
success criteria to be all the new features. I'm not sure that emphasizes
the point some were making in the call this morning, so please suggest more
text if you feel it is needed.

    Neil