BUG: loc with a multi-index and a list incorrectly selecting by position
from [SO](http://stackoverflow.com/questions/24056277/pandas-use-array-index-all-values)

This is a bug; the first should work like the 2nd (it is incorrectly selecting by position); essentially this is `.ix` fallback type behavior

```
In [10]: df = DataFrame(np.arange(12).reshape(-1,1),index=pd.MultiIndex.from_product([[1,2,3,4],[1,2,3]]))

In [11]: df.loc[[1,2]]
Out[11]: 
     0
1 2  1
  3  2

In [12]: df.loc[([1,2],),:]
Out[12]: 
     0
1 1  0
  2  1
  3  2
2 1  3
  2  4
  3  5
```
