WIP: Add AltAz to RaDec conversion tests against other packages

@eteq Here's a first attempt at a "verified" AltAz to RaDec conversion tests.

It doesn't work currently because the time chosen in that example is from 2041 ... I get this error:
```
E               IndexError: (some) times are outside of range covered by IERS table.
```

Apparently IDL and XEPHEM could compute this transformation ... what do I have to do to convince Astropy to do it too?

Do you have any other test cases from books, papers or tests in other software packages that we could put in here?