Unable to save parquet file using s3 url
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

data = [1, 2, 3]
df = pd.DataFrame()
s3_url = 's3://my_bucket/path/to/file.parquet'
df.to_parquet(s3_url)

```
#### Problem description

I am trying to upload a Panda's dataframe as a new file in s3. I am getting `FileNotFoundError` since the s3 file is opened for read instead of write. 
Flow:
```python
# frame.py:to_parquet():to_parquet(...)
# parquet.py:to_parquet():impl.write(...)
# parquet.py:write():get_filepath_or_buffer(...)
# parquet.py:get_filepath_or_buffer():s3.get_filepath_or_buffer(...)
# s3.py:get_filepath_or_buffer():fs.open(...)
# core.py():open():return S3File(mode='rb', ...)
...
```
It looks as if s3fs tried to read the file instead of write to it.

Another issue is that I can't specify AWS profile to use. See https://stackoverflow.com/questions/48486209/specify-aws-profile-name-to-use-when-uploading-pandas-dataframe-to-s3

#### Expected Output
A new file should be created.

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

<details>
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Darwin
OS-release: 16.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.22.0
pytest: 3.3.2
pip: 9.0.1
setuptools: 28.8.0
Cython: 0.27.3
numpy: 1.14.0
scipy: 1.0.0
pyarrow: 0.8.0
xarray: None
IPython: None
sphinx: None
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: 1.2.1
pymysql: None
psycopg2: None
jinja2: None
s3fs: 0.1.2
fastparquet: 0.1.3
pandas_gbq: None
pandas_datareader: None

</details>
