API: should .loc be as strict as .ix in tuple indexing?
from ML

https://groups.google.com/forum/?fromgroups=#!topic/pydata/fzPGjVrj9lU

```
df = pd.DataFrame.from_items([('a', [1,2]), ('b', ([3,4]))])
df.index = pd.MultiIndex.from_tuples([('A', 'B'),('C', 'D')],
        names=['k1', 'k2'])

# A dataframe, with a multiindex:
>>> df
       a  b
k1 k2
A  B   1  3
C  D   2  4
```

`.ix[('A','B')]` succeeds
`.ix['A','B']` succeeds as the `A, B` is tried as a tuple (`.ix` tries hard here)

propose that we change `.loc` to disallow the 2nd case and instead raise a KeyError
its more 'strict'
