drop() issue with MultiIndex after append()
Minimal example:

```
In [10]: df1 = pandas.DataFrame({'x': ['a', 'b'], 'y': ['A', 'A'], 'z': [1,1]}).set_index(['x', 'y'])

In [11]: df2 = pandas.DataFrame({'x': ['a'], 'y': ['B'], 'z': [1]}).set_index(['x', 'y'])

In [12]: df3 = df1.append(df2)

In [13]: df3
Out[13]: 
     z
x y   
a A  1
b A  1
a B  1

In [14]: df3.drop('a')
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
/home/me/<ipython-input-14-d69e3ae69a9d> in <module>()
----> 1 df3.drop('a')

/home/me/.local/lib64/python3.2/site-packages/pandas-0.8.1.dev_e1ff90c-py3.2-linux-x86_64.egg/pandas/core/generic.py in drop(self, labels, axis, level)
    298             new_axis = axis.drop(labels, level=level)
    299         else:
--> 300             new_axis = axis.drop(labels)
    301 
    302         return self.reindex(**{axis_name : new_axis})

/home/me/.local/lib64/python3.2/site-packages/pandas-0.8.1.dev_e1ff90c-py3.2-linux-x86_64.egg/pandas/core/index.py in drop(self, labels, level)
   1671                 inds.append(loc)
   1672             else:
-> 1673                 inds.extend(list(range(loc.start, loc.stop)))
   1674 
   1675         return self.delete(inds)

AttributeError: 'numpy.ndarray' object has no attribute 'start'
```

There isn't a duplicate index, so I think it should work.
