For initialisation from list of quantities, default to unit of first

This follows on a discussion on astropy-dev, where it was asked how to easily append quantities to a longer array. The best method so far would seem `Quantity([q1,q2], unit)`; this PR ensures that if no `unit` is given, it defaults to `q1.unit`:
```
w1 = u.Quantity(1000, u.AA)
w2 = u.Quantity(900, u.nm)
u.Quantity([w1, w2])
```
yields
```
<Quantity [ 1000., 9000.] Angstrom>
```
(Also includes a test case).

[1] https://groups.google.com/forum/#!topic/astropy-dev/zH98DFp0YKM