ENH Recognize 's3n' and 's3a' as an S3 address
This PR allows `read_csv` to recognize that "s3n://" designates a valid AWS S3 address. Partially addresses issue #11070 .

Fixed. I also realized (reading the links I added for issue #11070 ) that "s3a" is a valid S3 designator as well, so I added that too.
 The one test failure appears unrelated to the code change in this PR:

```
FAIL: test_numeric_column_names (pandas.io.tests.test_stata.TestStata)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/pydata/pandas/pandas/io/tests/test_stata.py", line 480, in test_numeric_column_names
    original.to_stata(path)
  File "/home/travis/miniconda/envs/pandas/lib/python3.3/contextlib.py", line 55, in __exit__
    next(self.gen)
  File "/home/travis/build/pydata/pandas/pandas/util/testing.py", line 2021, in assert_produces_warning
    % extra_warnings)
AssertionError: Caused unexpected warning(s): ['ResourceWarning'].
```
 no it's just hitting the actual error there
you have something failing
 add a whatsnew note in 0.17.0 (enhancements section)
 @jreback , green here too!
 thanks!
