backup should look, feel and behave more 'unixy'

i am using 'backup' for a couple of month now and absolutely love it. great tool, thanks for sharing it.
but i think, to use it properly on production servers it should behave more unixy.

just a couple of things:
- `config.rb`should be stored in `/etc/backup/config.rb`
- `$HOME/.backup/config.rb` should extend and overwrite the global configuration in `/etc`
- logs should go to `/var/log/backup.log` and i should be able to change the filename (not only the path)
- `logfile.max_bytes` should default to `Float::INFINITY` (see #31). logrotate does a pretty good job at handling logrotation, and is installed and configured on most systems anyway.
- global models should go into `/etc/backup/models` and user-models into `$HOME/.backup/models`
- `tmp_path` should default to somewhere in `/tmp`
- `data_path` should default to somwhere in `/var/` or `/etc/backup/` (i am not sure here, we can't use `/var/backup/` because it is used by debian based systems for internal backups)
- timestamps in logs and .yaml-files should use system-time, not UTC

this allows a sysadmin to set sane defaults (like mailserver, backupserver,...) systemwide, witch normal system users can use or overwrite.