Specifying axis label issue #27

@astrofrog - This change from ```Angle(np.asarray(values), unit=u.deg) ``` to ```Angle(values) ``` is causing one image test to fail in a strange way. I don't know why but the ticks for the x-axis are different for image_plot.png (only for this one axis and only on this one image).
![image_plot](https://cloud.githubusercontent.com/assets/3496036/3286499/ed37e490-f549-11e3-9a27-bd4d54a3bc85.png)
Reverting back to ```Angle(np.asarray(values), unit=u.deg)``` makes all the tests pass..