let the PathService be usable without a ServletRequest

This small patch lets the PathService be used from outside the filters of seraph.
For instance, I wrote those simple jsp tags which make it easy to output "secured" links. (ie, the <a>link</a> is output only if the user has access to the path given in the href attribute) See https://swaf.dev.java.net/source/browse/swaf/swaf-tags/seraphtags/src/java/net/incongru/security/seraph/taglib/