#382 Add HTML tags to titles and legends

[TheDougMiester]

To properly display scientific charts, it's desirable to have superscripts and subscripts of various letters not represented in the Unicode set.

It has come to my attention that JFreeChart has already been modified to support this feature in another Sourceforge project, but hasn't been turned over as a fix here. The name of the other project is jparsec (an astrophysics package, NOT the word parser with the same name).

For science apps, it would be really, really great to have HTML tags on chart titles and axis legends