Add rules to mtable to properly handle displaystyle and scriptlevel as described in the MathML spec.  Resolves issue #839.
Don't do the scriptlevel adjustment, as per discussion on w3c math list.
Make displaystyle and scriptlevel be forced only if they are in the list of default properties.  Resolves issue #912 better than before.
Merge branch 'issue839' into develop.  Issue #839.
Merge branch 'issue912' into develop. Issue #912.