Add describe_db_log_files request for AWS, version 2013-05-15
Add 'each' that iterates over all log files

Use 'select' to do an exact match when seeking a specific log file.
Add parser/request for DownloadDBLogFilePortion
Use correct filters in RDS model logfiles 'all' method
More DRY, succinct implementation of tag parsing
Merge branch 'feature/AWS_RDS_Download_DB_LogFile_Portion'
Remove extraneous nil
Add db identifier to parser, use it for log_file model
Get partial log content via method call; add to log_file model attributes
Merge pull request #1949 from jbence/feature/AWS_RDS_Describe_DB_LogFiles

[aws|rds] implement describe_db_log_files/download_log_file_portion