add new Credentials class

Reading/writing the credentials should be handled by a specific class, this PR also adds some improvement to the original code:

- created `Credentials` class (grouped read/write functions together)
- security: the credentials files are written with 0600 permissions (i.e. `root` only)
- security: redefined `#to_s` to avoid logging the password

Logging is disabled for now, it needs some refactoring in `SUSE/connect` first.