Download to stdout & cat command

[mjurbanski-reef]

• Add cat command for downloading file contents directly to stdout
• Emit Using https://api.backblazeb2.com message to stderr instead of stdout, therefor prevent JSON output corruption
• Stream ls --json JSON output instead of dumping it only after all objects have been fetched
• Alias - to stdout in download-file-by-name or download-file-by-id command

Internal changes:

• refactor integration tests a bit
• replace some duplicated b2sdk code with imports from b2sdk