Add ability to include form auth password in error page via FormAuthValve

There should be optional support for including the j_password form auth parameter in the session by adding an attribute to the FormAuthValve. For example:

<!-- Add the FormAuthValve to get access to the exception -->
<Context cookies="true" crossContext="true">
<Valve className="org.jboss.web.tomcat.security.FormAuthValve"
includePassword="true" />
</Context>