beamerthememjo.sty: add footnote handling

beamerthememjo.sty: add footnote handling

Use symbols for the footnotes, and reset the counter on each frame.
Using numeric indicators can be confusing when they appear next to a
number (such as a year) because they look like exponents. We also do
not need to preserve the counter across slides; the audience can see
only one slide at a time. Finally, we skip the asterisk (the first
default symbol) because it is ugly / wide.