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
I saw a similar comment in the test function `imageplot`. That one should also get removed or better split into two test functions.
It might be a nice goal to have a test case for each option that matlab2tikz offers.