Add built-in authentication/authorization support for Seam Security

As part of the solution for MODE-1270, it would also be useful for ModeShape to have built-in support for Seam Security, which would allow logging into a Repository without a Credentials by obtaining and using the current Seam Security Identity (via "Identity.instance()"). Obviously this authentication provider would be enabled only when the "org.jboss.seam.security.Identity" class is on the classpath.

Whereas MODE-1270 addresses using JAAS within J2EE applications, obtaining the Subject within a J2EE container is difficult and non-standard. By providing this alternative, ModeShape is better able to integrate seamlessly (ha, see what I did there?) in the J2EE world (especially within the JBoss AS ecosystem).