Add options for constructing clients & resources.

This pull request adds all of the options available through Botocore's
client constructor for making Boto 3 clients and resources, including
credentials, region, SSL, and other settings.

```python
sqs = boto3.resource('sqs', region_name='us-west-2')

s3 = boto3.resource('s3', aws_access_key_id='YOUR_KEY',
                    aws_secret_access_key='YOUR_SECRET')
```

It also removes the default of `'us-east-1'` and requires you to set
a region. The docs have been updated (slightly) and the tests have
been updated to reflect this. I plan to flesh out the configuration
documentation in the future.

cc @jamesls, @kyleknap