Fix two bugs due to typos in compat.py

This pair of patches repairs two bugs in `compat.py` that are probably due to typos.

Here, in `cal_date`, the local variable `day` shadows the input argument of the same name:

    def cal_date(day):
        ...
        year = c_short()
        month = c_short()
        day = c_short()
        hour = c_double()
        ...
        _cal_date(day, byref(year), byref(month), byref(day), byref(hour))

        return year.value, month.value, day.value, hour.value

This is fixed in the first patch by renaming the input argument to `jd`.

Here, in `limb_angle`, `_limb_angle` is supposed to be called, but the extraneous `=` instead makes the code in parentheses a tuple that is assigned to `_limb_angle`.

    _limb_angle = ((c_double*3)(*pos_obj), (c_double*3)(*pos_obs),
                   byref(limb_ang), byref(nadir_ang))

    return limb_ang.value, nadir_ang.value

This is fixed in the second patch by removing the `=`.

I maintain a [MacPorts package of python-novas](http://www.macports.org/ports.php?by=name&substr=novas_py), and these patches are [currently part of that package](https://trac.macports.org/browser/trunk/dports/python/py-novas_py/files/patch-compat.py.diff).