[eit] add --pretend support to push, pull and cleanup commands
[entropy.server] add "weak package files" support

Weak package files is a feature that makes possible to save a lot of
local space if enabled. Basically, all the package files no longer in
the repository are locally removed before their expiration. This reduces
local data redundancy with no real reliability impact.
Release Entropy 179
Tagging Entropy version 179
[entropy.server] fix typos in tidy_mirrors()
[entropy.server] s/extra_package_rel/package_rel/ in tidy_mirrors()
[entropy.server] tidy_mirrors: do not show packages as expired if they've already marked
[entropy.server] tidy_mirrors: do not show packages as weak if they've already marked
[entropy.server] tidy_mirrors: fix weak file detection logic
[entropy.server] _show_sync_queues: inform about package download as well
[entropy.server] _calculate_local_package_files: take into consideration .weak files
[entropy.server] _calculate_local_package_files: properly handle .weak files