remove obsolete comment (now that we hookMath() before firepad loads any content).

There might be an issue of duplicate markText() for same area if user edits during renderAllMath() but that'd be a CM-MJ bug.
Use same header sizes as browser H1..H6.
Drop line-height because it doesn't work well with wrapped headers and there is a better fix (see #16).
Add some vertical padding to heading lines.  Closes #16.