Add celestial conversion check for different epochs of observation

[cdeil]

Like @dsberry said in #4 (comment) :

2. In the table, for the "Epoch of observation", you probably want to use
   the same value (J2000) for all systems, at least initially. Having said
   that, you have a problem if any of the coordinate packages do not allow the
   epoch of observation to be specified. For instance, I'm not sure what the
   current implementation of astropy coords assumes about the epoch of
   observation. I understand Erik has agreed that an ObsTime property is
   needed, but I presume he hasn't yet added it.

3. A further development could be to add more tag names for other
   combinations, like "fk5_J2010_J2012.34" meaning equatorial fk5 with respect
   to equinox of J2010, observed at J2012.34. Converting to and from systems
   with non-default values for equinox and epoch should test more bits of the
   code. Although of course the total number of tests could become difficult
   to handle.

I agree we should keep the number of tests small.
We could just test astropy (once obstime is available) against pyast and I don't think we need all systems here.

[astrofrog]

I think we should start thinking about adding a test or two for arbitrary equinox and epoch of observation. For example, at the moment, I'm testing the following with Kapteyn to check the implementation of FK4 -> FK5 in Astropy

before = (celestial.equatorial, celestial.fk4_no_e, "B1930", "B1966_OBS")
after = (celestial.equatorial, celestial.fk5, "J2030")
coords = celestial.sky2sky(before, after, 45., 46.)

@cdeil - I'm not sure how to best do this, because as you were saying above, we should keep the number of tests small. Maybe we could just compare against kapteyn and pyAST? (or even just PyAST)