Two new Pull Requests to review - refactor Main and fix ResourceWarning

Hi,

as the subject says:

Refactor Main.py into the actions' run() methods
#550 opened 1 hour ago by ericzolf
https://github.com/rdiff-backup/rdiff-backup/pull/550

Get rid of spurious resource warnings
#549 opened 2 hours ago by ericzolf
https://github.com/rdiff-backup/rdiff-backup/pull/549

After that, I'm thinking about creating an alpha version so that people
(aka you) can try the new interface and make sure that everything
remained backward compatible with v2.0.5.

This written, it makes only sense if you take the time to test this
alpha version properly. Opinions?

Thanks, Eric