K2ephem

This release ensures we use `https://ssd.jpl.nasa.gov` instead of `http://ssd.jpl.nasa.gov` (i.e. *https* instead of *http*).

This version of K2ephem makes it easier to check the visibility of comets. The "CAP" directive is now automatically added for comets, which tells JPL Horizons to automatically select the appropriate apparition.

- Adds a convenient `get_ephemeris_dataframe` function to obtain the ephemeris for an object for a given campaign as a Pandas dataframe.
- Use JPL Horizon's astrometric rather than apparent coordinates, because stellar aberration is corrected downstream in the observing process.
- Minor changes in style and docs.

- the magnitude and angular speed are now displayed for objects that pass through the field;
- an optional `--plot` argument was added to create rudimentary plots showing the path of an object with respect to each K2 field.

Release notes
- Bugfix release: the MANIFEST.in file was missing, causing installs from PyPi to fail.