Pass minimal flake8 linting.

Tabs to spaces, no trailing whitespace. From my syntastic flake8 config, these warnings are ignored:

    E501,E302,E261,E701,E241,E126,E127,E128,E121,W801
Allow the location of tarsnap to be configurable.
Delete old daily backups just like we do with weekly ones.
Merge branch 'specify-tarsnap-path' into delete-old-daily-backups