Mirrors

Adds support for source file mirrors.

They can be specified in portmod.cfg via the PORTMOD_MIRRORS variable, or if unspecified the default mirror (the portmod/mirror repo) is used.

Also added a mirror script which downloads any missing sources (that don't have RESTRICT="mirror" or RESTRICT="fetch") into the specified directory to help automate mirror updates.

Downloads are now wrapped in a progressbar (closes #26 (closed)), and mirrors are tried prior to the original download locations (unless RESTRICT="mirror" is set).

There are some import changes wrapped into this, as I ran into some cyclic imports when implementing this.

I also typed and documented the remainder of portmod.repo.download.py.