Implement the changes proposed in issue #28

This patch defines default values for transaction types and their weights in `plugin.xml` (See issue #28). The values can be still overriden in the actual config file.

Also adds a --dbconfig optoin (shortcut -d) where you can specify the database connection information.

See `config/mix_config_postgres.xml` for the lean config example. If you like the look of it, I could adjust other sample configs, too.