Make sure infrastructure files get excluded from coverage testing, and ported over fix to edit-on-github feature from astropy affiliated package template.
Remove unused code
Exclude six.py from coverage tests
Merge pull request #59 from astrofrog/infrastructure-fixes

Make sure infrastructure files get excluded from coverage testing
Updated README.md
Tick angles point along gridlines regardless of aspect ratio

I had previously computed the tick angle by advancing a fixed distance
in axes coordinates. This was wrong, because the axes coordinates always
run from 0 to 1, regardless of aspect ratio. The solution in this patch
is to compute the offsets by advancing in figure coordinates (i.e.,
pixels).

Fixes #62.
Merge pull request #63 from lpsinger/tick_angle_non_square_axes

Tick angles point along gridlines regardless of aspect ratio
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)
Add unit test