Make SolarSysObject.ephemeris take an observer, rather than a target.

This makes ephemeris similar to lightcurve and fluxd.  Only observe is
different, but in this case the Object is observing something else.
Merge pull request #18 from mkelley/ephemeris-refactor

Ephemeris to take an observer, rather than a target.
New ephem.getspiceobj.
Fix test_ephem for new ephemeris behavior.