Panel.to_excel fails when axis 0 contains unicode strings
Hello,
while exporting a panel with unicode strings in the items axis `to_excel` fails with a `UnicodeEncodeError`. The following code reproduces the error:

``` python
import pandas as pd
p = pd.Panel({u'Smørrebrød': pd.DataFrame({})})
p.to_excel('nope.xlsx')
```

I suspect that changing `name = str(item)` to `name = unicode(item)` around line 440 in `pandas/core/panel.py` fixes this small issue.

Thanks!

``` python
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.9.final.0
python-bits: 64
OS: Darwin
OS-release: 14.0.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.15.2
nose: 1.3.4
Cython: 0.21.2
numpy: 1.9.1
scipy: 0.14.0
statsmodels: None
IPython: 2.3.1
sphinx: None
patsy: None
dateutil: 2.2
pytz: 2013b
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.4.2
openpyxl: None
xlrd: None
xlwt: 0.7.5
xlsxwriter: 0.6.6
lxml: 3.4.1
bs4: 4.3.2
html5lib: 0.999
httplib2: None
apiclient: None
rpy2: None
sqlalchemy: 0.9.8
pymysql: None
psycopg2: 2.5.4 (dt dec pq3 ext)
```
