Several new features

[leandro-lucarella-sociomantic]

This PR adds several new features:

• Dry-run mode, to see what it will be done before actually doing it
• Add support for OAuth authentication
• Properly close copied issues when the original was closed
• Add support to "move" issues (meaning, the original issue will be closed if it was opened, and adding a comment saying the issue was moved in any case).

Other small fixes are included too:

• Load all the milestones, not only open ones
• Add support for custom verbs to send_request(), this was necessary to update issues (close them)
• Don't ask questions when not in a tty (this allows piping the program using xargs, for example, to get a list of issues to import)
• Remove trailing white spaces

I know including many features in one PR might be a bit suboptimal, given that all have to be accepted (or rejected) together, but splitting it in several PR could be some work, so I would prefer to do it only if necessary.