Some docstrings indicate str return value when they should indicate bytes

[mattsb42-aws]

As raised in issue 6, some of the docstrings still indicate a str return value. The docs need to be swept to catch any of these cases[1][2] and fix them.

[1] https://github.com/awslabs/aws-encryption-sdk-python/blob/master/aws_encryption_sdk/__init__.py#L61
[2] https://github.com/awslabs/aws-encryption-sdk-python/blob/master/aws_encryption_sdk/__init__.py#L106

[mattsb42-aws]

This is addressed in #12