Class::DBI::Plugin is an abstract base class for Class::DBI plugins. Its
purpose is to make writing plugins easier. Writers of plugins should be able
to concentrate on the functionality their module provides, instead of having
to deal with the symbol table hackery involved when writing a plugin module.
Only three things must be remembered:
* All methods to be exported are given the "Plugged" attribute. All other
methods are not exported to the plugged-in class.
* Method calls that are to be sent to the plugged-in class are put in the
init() method. Examples of these are set_sql(), add_trigger() and so on.
* The class parameter for the init() method and the "Plugged" methods is the
plugged-in class, not the plugin class.
