Automate database upgrades

Issue #120

Provide a new program to ease database upgrades:
    $ netmagis-dbupgrade [-f netmagis.conf] [target]

-f : use alternate netmagis.conf file

If no target is specified, the program displays:
    - detected database schema version
    - highest supported database version
    Example:
	$ netmagis-dbupgrade -f /etc/netmagis.conf.new
	Detected schema version: 21
	Latest schema supported: 22
If a target is specified, the database will be upgrade
to this target version:
    Example:
	$ netmagis-dbupgrade -f /etc/netmagis.conf.new 22
	Preparing upgrade from 21 to 22.
	Upgrading: 21-22
	Upgrade successful