crosstabs doesn't work with margin and normalize together
#### Code Sample, a copy-pastable example if possible
```python
import pandas as pd
df = pd.DataFrame({"A": ["foo", "foo", "foo", "foo", "foo",
          "bar", "bar", "bar", "bar"],
                    "B": ["one", "one", "one", "two", "two",
                          "one", "one", "two", "two"],
                    "C": ["small", "large", "large", "small",
                          "small", "large", "small", "small",
                          "large"],
                    "D": [1, 2, 2, 3, 3, 4, 5, 6, 7],
                    "E": [2, 4, 5, 5, 6, 6, 8, 9, 9]})

pd.crosstab([df.A,df.B],df.C,margins=True,margins_name='Sub-Total',normalize=0)
```
#### Problem description

pandas.Crosstab:
As per the Documentation, For parameter normalize If margins is True, will also normalize margin values. However when I give normalize as True and margins as True with margin_names as a string then it throws following exception:

KeyError: "['Sub-Total'] not found in axis"

where Sub-Total is the margins_name String.

#### Expected Output
Normalize value of the Margin rows and columns

#### Output of ``pd.show_versions()``


INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 142 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.24.2
pytest: 3.2.1
pip: 18.0
setuptools: 39.1.0
Cython: 0.26.1
numpy: 1.15.4
scipy: 1.0.0
pyarrow: 0.8.0
xarray: 0.10.7
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.1.0
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml.etree: 4.1.0
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: 2.7.5 (dt dec pq3 ext lo64)
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None
None

