[issue-441] replace print() with logging

[armintaenzertng]

fixes #441

[meretp]

Your changes look good to me. I found two additional places in the codebase where information should bel logged. As I can't comment on unchanged lines, I reference them here:

• the output to the console is a logging.info
• the system exits here should also be logged

Within the parser I also implemented a Logger- class, I didn't look in the documentation for the logging package but do you know if we could merge this somehow? So does the logging package provide any options for an individual logger? Although I am not sure if we would mix different levels of logging when merging this.

[armintaenzertng]

I fixed the system exits' logs, but for the cli tool parser the output is the actual wanted result of the tool and should therefore go to stdout, I think.

[meretp]

Thanks for addressing my remark and I agree that the output is wanted as is.
Do you have any thoughts on my last question concerning an indiviual logger?

[armintaenzertng]

Concerning the parsing logger, which basically just collects all parsing errors, I think that it is still valid as it is to give helpful information for the SpdxParsingError that we raise (this can be caught by library users, which can easily do whatever they want with the get_messages() function).
The question would be if we additionally want to separately log all these errors. But right now, I don't see the merit in that.