Default implementation of the base Handler interface. Simply stores a reference to the ServletContext and offers
access to the previously-registered Handler of the same type (the one that comes before the current one in the
list under <pageflow-handlers> in beehive-netui-config.xml).
Initialize.
Parameters: handlerConfig - the configuration object for this Handler. Parameters: previousHandler - the previously-registered Handler, which this one can adapt. Parameters: servletContext - the ServletContext for the webapp that is creating this object.