better quantities support
I want to use the quantities package and pandas to process scientific data.  However, pandas strips the unit(s) of the data stored in numpy arrays if I create a dataframe out of them:

``` python
a = np.random.rand(10)*pq.s
b = np.random.rand(10)*pq.A
df = pd.DataFrame({'current':b, 't':a}, columns=['t','current'])
```

``` python
In [1]: df
Out[1]: 
         t   current
0  0.663397  0.435423
1  0.038498  0.101763
2  0.960983  0.091785
3  0.262863  0.364734
4  0.154440  0.274169
5  0.953129  0.052678
6  0.389961  0.272535
7  0.961604  0.559451
8  0.747192  0.438268
9  0.789207  0.568685

```

What do you think,  should pandas have support for writing the unit of each column in the corresponding columnname if the column is a quantities array,  or should it be part of the quantities package.

``` python
In [1]: df
Out[1]: 
       t [s]  current [A]
0  0.663397  0.435423
1  0.038498  0.101763
2  0.960983  0.091785
....
```
