Panel.to_excel() doesn't allow conditional formatting after the fact. 
Trying to add conditional formatting to an excel file written from pandas using pd.ExcelWriter and...

This doesn't work:

``` python
# results is a dictionary of DataFrames
data = pd.Panel(results)
writer = pd.ExcelWriter(file_name+".xlsx", engine='xlsxwriter')
data.to_excel(writer)
for worksheet in writer.sheets.values():
    worksheet.conditional_format('E2:E38', {'type': '2_color_scale'})
writer.save()
writer.close()
```

And this does:

``` python
# results is a dictionary of DataFrames
writer = pd.ExcelWriter(file_name+".xlsx", engine='xlsxwriter')
for k,v in results.iteritems():
    v.to_excel(writer, sheet_name=k)
for worksheet in writer.sheets.values():
    worksheet.conditional_format('E2:E38', {'type': '2_color_scale'})
writer.save()
writer.close()
```

Is something not implemented in Panel structure? or is this a design decision along the lines of what causes issue #8553?
## INSTALLED VERSIONS

commit: None
python: 2.7.10.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 69 Stepping 1, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.17.0
nose: 1.3.7
pip: 7.1.2
setuptools: 18.4
Cython: 0.23.4
numpy: 1.9.3
scipy: 0.16.1
statsmodels: 0.6.1
IPython: 4.0.0
sphinx: 1.3.1
patsy: 0.4.0
dateutil: 2.4.2
pytz: 2015.7
blosc: 1.2.8
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.4.4
matplotlib: 1.5.0rc3
openpyxl: None
xlrd: 0.9.4
xlwt: None
xlsxwriter: 0.7.7
lxml: None
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.9
pymysql: None
psycopg2: None
