ENH: json_normalize should allow a different separator than .
```python
>>> import pandas
>>> col_in = ['c1', 'c2.x']
>>> df = pandas.DataFrame([['A', 0], ['B', 1]], columns=col_in)
>>> df.c1
0    A
1    B
Name: c1, dtype: object
>>> df.c2.x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/core/generic.py", line 2744, in __getattr__
    return object.__getattribute__(self, name)
AttributeError: 'DataFrame' object has no attribute 'c2'
```
#### Problem description

The above snippet shows that it's not ideal to have `.` as a character in a column name. (I'm running into this when using Vega for data visualization, https://github.com/vega/vega-lite/issues/1775.) When `json_normalize` flattens a nested input JSON, it separates the nesting levels with a `.`. I believe this happens on this line:

https://github.com/pandas-dev/pandas/blob/7d8bc0deaeb8237a0cf361048363c78f4867f218/pandas/io/json.py#L831

I'd like to see an additional argument to `json_normalize`, `separator`, with default `.`, that specified the character (string) that separated nesting levels. In the line of code above, `'.'.join(val)` would be replaced by `separator.join(val)` (if I'm reading what that line does correctly). I could use, say, `_` to use underscore instead of period.

n00b at pandas, please correct me if I'm doing anything wrong.

#### Expected Output

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

<details>
>>> pandas.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Darwin
OS-release: 16.3.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.US-ASCII
LOCALE: None.None

pandas: 0.19.1
nose: 1.3.7
pip: 9.0.1
setuptools: 30.3.0
Cython: 0.25.2
numpy: 1.11.2
scipy: 0.18.1
statsmodels: None
xarray: None
IPython: 5.1.0
sphinx: 1.5.1
patsy: None
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: 3.2.3.1
numexpr: 2.6.1
matplotlib: 1.5.3
openpyxl: 2.4.1
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: 3.6.0
bs4: 4.5.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None
</details>
