This changes the behavior of environment variable credential loading to
function as described in aws/aws-cli#113. Here is what this looks like
for both the CLI and Botocore/Boto 3:

CLI:
```bash
$ AWS_ACCESS_KEY_ID=foo AWS_SECRET_ACCESS_KEY=bar aws s3 ls

$ AWS_ACCESS_KEY_ID=foo AWS_SECRET_ACCESS_KEY=bar aws --profile dev s3 ls
```

Python:
```python
$ AWS_ACCESS_KEY_ID=foo AWS_SECRET_ACCESS_KEY=bar python
>>> import boto3
>>> boto3.setup_default_session(profile_name='dev')
>>> # The following will use the profile, not the env vars!
>>> s3dev = boto3.resource('s3')
```

Added a test and a new log message to ensure it's obvious what is happening.