Bugfix for multilevel columns with empty strings in Python 2
 Allow unicode empty strings to be used as placeholders in multilevel column names in Python 2.

The code in `Frame._getitem_multilevel()` only treats empty strings as placeholders in multilevel columns if they are of type `str`. Since in Python 2 this means bytestrings, this means a unicode empty string is not treated as a placeholder. This is a problem for Python 2 code that is trying to be forward-compatible with Python 3 by importing `unicode_literals` from `__future__` and using empty strings in DataFrame construction - those empty strings will be unicode strings and not treated as placeholders by pandas.

This patch changes the logic to allow anything that passes an equality check with `''` to be used as a placeholder, rather than checking its type, which means that after this patch either bytestring or unicode empty strings can be used as placeholders in Python 2, and the situation is unchanged in Python 3 (unicode strings, but not bytestrings, will be treated as placeholders).

Without this patch, the following code:

```python
from __future__ import print_function
import pandas

# Unicode string placeholder in multilevel column:
df = pandas.DataFrame({(u'test', u''): [1.0, 2.0, 3.0]})
print(df['test'])
print(type(df['test']))
print()

# native string type placeholder in multilevel column:
df = pandas.DataFrame({(u'test', ''): [1.0, 2.0, 3.0]})
print(df['test'])
print(type(df['test']))

```
Results in the following in Python 2:

```

0  1.0
1  2.0
2  3.0
<class 'pandas.core.frame.DataFrame'>

0    1.0
1    2.0
2    3.0
Name: test, dtype: float64
<class 'pandas.core.series.Series'>
```

Note the first blank line in the output when a unicode empty string placeholder was used. It's an empty string column label for the dataframe.

Whereas in Python 3 it results in:
```
0    1.0
1    2.0
2    3.0
Name: test, dtype: float64
<class 'pandas.core.series.Series'>

0    1.0
1    2.0
2    3.0
Name: test, dtype: float64
<class 'pandas.core.series.Series'>
```

After the patch, the Python 2 result is the same as the Python 3 result.