WIP: Ephemerides right

This is a work in progress PR to tackle #472 where I'm trying to - step by step - provide a properly tested (ie. with external values) the equation used for the ephemerides in SunPy.

At the moment, the test will fail mainly due that the others function doesn't accept the units that in this case mean_anomaly is producing.  Once pull-request #1030 is accepted this should pass the test too.