Refactor Invoice#copy by moving logic from controller to model.
Guess period when copying invoice.
Couple of gem updates.
Update rubyzip and selenium-webdriver.
Implement support for upgrades from older backups.

This is done by using a zip file as backup. This includes both the YAML
data and a schema.rb. When restoring, we create an empty database using
the schema.rb, import the data, and then run all the pending migrations.
Merge branch 'features/backup_with_schema'
Update CHANGELOG.