Plot degree labels in TeX mode

Use Unicode character or TeX math mode depending on whether `tex.usetex`
is enabled. Avoids exceptions like the following:

    UnicodeEncodeError: 'ascii' codec can't encode character u'\xb0' in
    position 290: ordinal not in range(128)
Merge pull request #58 from lpsinger/tex_labels

Plot degree labels in TeX mode