Fix shadowed variable in cal_date

The input argument, day, was shadowed by the ctypes object, day.
Renaming the input argument to jd fixes this.
Fix unintended assignment in limb_angle

Due to a typo, an extraneous '=' assignment operator, rather than
calling _limb_angle, the code assigned a tuple to _limb_angle.
Merge pull request #1 from lpsinger/compat_fixes

Fix #1 involving two typos in compat.py