Second attempt at the Ipacwriter @keflavich started

Almost done. This stuff works, and it round-trips the tabes I tried.
It could take some more tests though to check that the tests that test the compatibility with the IPAC format definitions raise the right Exceptions.

@keflavich sorry for stealing your pull requests here. I opened a new one against astropy/master so the Travis runs on it.
Do you want to take over from here again?

@taldcroft comments?

closes #833