[aws|rds] implement describe_db_log_files/download_log_file_portion

This change provides access to the describe_db_log_files and download_log_file_portion requests for AWS RDS, api version 2013-05-15.

The log file content is added as a merged attribute (along with other attributes that determine if more log content is available). The download_log_file_portion API call uses the 'Marker' parameter in a slightly different way than other API calls. I think the approach of augmented the log_file model with extra attributes is the most natural way to handle working with the log file content, as opposed to introducing an additional model.

I have attempted to include tests but I am not certain of the actions that would be taken against our AWS account so I have not run the suite. I wrote my own small script (not committed, of course) to verify that the implementation is working.

I'd appreciate feedback on the tests (and the rest of it, naturally).