Possibility to have logOutPage URL configurable

There is declared field "logOutPage" on BaseFormAuthenticator, which points to URL where user will be redirected after global logout. Currently field is hardcoded to value "/logout.jsp".

It will be good to have it configurable, so that people can change logoutURL in context.xml configuration like:

  <Valve className="org.picketlink.identity.federation.bindings.tomcat.sp.SPRedirectFormAuthenticator" 
	 logOutPage="/" />

Default value should still remain to "/logout.jsp" to ensure backwards compatibility.