update AUTHORS.
update to librepo API 1.2
move format_number, format_time to dnf.cli.format

We should not put low-level functions in the same module
(same class in this case) as the high-level setup code,
as adding mid-layer code to a different module causes
circular dependencies.
drop per-repo interrupt callback

This is ignored in repo.set_interrupt_callback() and with multi-downloader
there's no "active" repo anyway. Callbacks (and progress meters, too)
must be global.
add dnf.cli.progress

MultiFileProgressMeter: this meter is used to display progress
of multiple downloads running in parallel.

LibrepoCallbackAdaptor: drop-in replacement of the old callback
adaptor for single-file downloads. The win here is we don't have
to import urlgrabber.progress anymore.
add unit tests
download_packages() cleanup
use librepo.download_packages()