Distance should accept lists of quantities with different units

Like Quantity and now Angle, Distance should work for the following case:

```
In [4]: Distance([3 * u.kpc, 3 * u.Mpc])
---------------------------------------------------------------------------
UnitsError                                Traceback (most recent call last)
<ipython-input-4-8ee9337cc5d3> in <module>()
----> 1 Distance([3 * u.kpc, 3 * u.Mpc])

/Users/tom/Library/Python/2.7/lib/python/site-packages/astropy-0.4.dev7744-py2.7-macosx-10.8-x86_64.egg/astropy/coordinates/distances.pyc in __new__(cls, value, unit, z, cosmology, distmod, dtype, copy, allow_negative)
    141                              'in Distance constructor')
    142         elif unit is None:
--> 143             raise u.UnitsError('No unit was provided for Distance')
    144         #"else" the baseline ``value`` + ``unit`` case
    145 

UnitsError: No unit was provided for Distance
```

cc @mdboom @mhvk