add epoch and equinox to sources table

[kelle]

Per #72 : In order to be able to astropy.coordinates to predict coordinates of objects at any date, we need epoch and equinox of the coordinate in the sources table.

[dr-rodriguez]

We probably want to store equinox as a string (eg, J2000), but for epoch, do we want to store it as floats (2015.0), strings ('2015-01-01'), or datetime.Date(or DateTime) objects?
Strings are probably easiest, but Date might allow for more powerful searches- if that were a use case.
For proper motion calculations, this would have to be converted to astropy.time.Time objects regardless.