fix crash on unicode level names
This fixed my problem when calling DataFrame.stack(<integer level number>). Not really familiar with pandas internals, but couldn't find any reason why unicode strings won't be accepted here.

tests would be needed
 To be honest, I don't know pandas internals/tests and I don't use python3. Please consider this as a bug report. Let me know if you need help with reproducing it.
 @alavrik 

can you show some code which reproduces the error?

the point of a bug report is to say, here is what I did, here's how to reproduce it, its doing x, but I think it should by doing y

we cannot simply change code because it 'looks right'. then what happens when someone decided that some other way looks right?

this is not about knowing pandas internals or not, its about giving a bug report that can lead to tests that can lead to fixed code

sorry for being harsh, but I find lots of people writing code w/o even thinking about testing it.
 @jreback 
You are right. Here's the test (Python 2.7.5, Pandas 0.15.2):

``` python
import pandas as pd
i = pd.MultiIndex(
    levels=[[u'foo', u'bar'], [u'one', u'two'], [u'a', u'b']],
    labels=[[0, 0, 1, 1], [0, 1, 0, 1], [1, 0, 1, 0]],
    names=[u'first', u'second', u'third'])
s = pd.Series(0, index=i)
s.unstack([1, 2]).stack(0)
```

and this is the last couple of frames of the error output:

```
/Users/alavrik/devel/ipython/ve/lib/python2.7/site-packages/pandas/core/index.pyc in sortlevel(self, level, ascending, sort_remaining)
   3868         if isinstance(level, (str, int)):
   3869             level = [level]
-> 3870         level = [self._get_level_number(lev) for lev in level]
   3871 
   3872         # partition labels and shape

/Users/alavrik/devel/ipython/ve/lib/python2.7/site-packages/pandas/core/index.pyc in _get_level_number(self, level)
   3166         except ValueError:
   3167             if not isinstance(level, int):
-> 3168                 raise KeyError('Level %s not found' % str(level))
   3169             elif level < 0:
   3170                 level += self.nlevels

KeyError: 'Level t not found'
```

It is easy to see what's going on. Instead of iterating over `[level]`, it is trying to iterate over the characters of the unicode `level` name. In this case, it starts with the character `t` of the level `two`.
 closing in favor of #9856 bug report
