Jplephem/add earth

@brandon-rhodes - as I have been using jplephem to do barycentering, I thought I should pass on the two corrections that make this easier (for me at least). One is to add `'earth'` to the recognized names, which causes coordinates of `'earthmoon'` and `'moon'` to be added appropriately. The other is to allow two-part `tdb` input, which solves the minor precision loss I mentioned on the astropy-dev mailing list.  Thanks for the having the neat, all-python, and very readable package! -- Marten