__init__.py cleanup: top-level AUTHORS file; option parsing in separate file

## Cleanup of `youtube-dl/__init__.py`

* the list of contributors is now in a separate file, `AUTHORS`. Why? Because this is meta-information which applies to the entire project, not just one particular module. Also, this file can and should be extended to indicate the authors' different roles (owner, maintainer, contributor, etc). Many successful projects follow this layout, e.g. [django](https://github.com/django/django/blob/master/AUTHORS) and [Swift](https://github.com/openstack/swift). We should adopt it as well.

* Argument parsing has been extracted to `configuration.py` and the names have been changed to follow PEP-8. Unnecessarily complicated boolean conditions have been simplified

* unused imports have been removed

## Test and review
I did my best not to introduce any bugs in the process, but it doesn't seem we have any tests at all for the command-line interface and option parsing, so this will require thorough review! I'm looking forward to any comments and improvements, and please let me know if I broke `devscripts/bash-completion.py` or not.