[AstroPy] Trouble when creating a vectorial Quantity from its components

[Juan Luis Cano]

Hello all, I'm using astropy 0.3 under python 3.3 with numpy 1.8.0, and 
while I was trying to build a vector `Quantity` from its components I 
found this:

```
 >>> import numpy as np
 >>> from astropy import units as u
 >>> p = 1000 * u.km; p
<Quantity 1000 km>
 >>> nu = 28 * u.deg
 >>> r = [p * np.cos(nu), p * np.sin(nu), 0]; r
[<Quantity 882.947592858927 km>, <Quantity 469.4715627858908 km>, 0]
```

This is perfectly normal, but I expected a vectorial `Quantity`. Trying 
to convert to an array didn't work:

```
 >>> np.array(r)
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
ValueError: setting an array element with a sequence.
```

Is there any way I can avoid doing this?

```
 >>> r = [p.si.value * np.cos(nu), p.si.value * np.sin(nu), 0] * 
p.si.unit; r
<Quantity [ 882947.59285893, 469471.56278589,      0.        ] m>
```

because it might get quite verbose if there are more magnitudes.

Thanks!