Fix #402 special characters

* Fix conversion of degree smybol "°"
* Correct handling of "<" and ">" in text mode (`parseStringsAsMath` is disabled)
* Add handling of ">>", meaning "much greater" if `parseStringsAsMath` is enabled
Here, I would be cautious to prevent `double superscript` errors from happening, i.e. `x^n^\circ` would trigger that (or if there is a superscript after our `^\circ`). The superscripts have to be grouped explicitly, and I don't know whether our code is smart enough to do so? Could you check this?
If it doesn't, the first work-around that comes to my mind is to box that `\circ`in, e.g. `{{}^\circ}`. That cannot produce double superscripts as far as I see, but I have to admit that TeX code looks fugly. 

Probably, there are only a few people that would use superscripts right next to a degree symbol, though.