XML Validation of subsystems

add JBossEntityResolver to resolve imported schema

* XML validation of remoting and threads subsystem
* logging XML can not be validated due to the use of @@ placeholders (the XML validation does not take into account any supplement's replacement)

The pending commits for the wildfly project are at https://github.com/jmesnil/wildfly/commits/xsd_validation_subsystems. I'll open a PR for wildfly with them once wildfly-core 1.0.0.ALPHA15 is released (as some subsystem requires to resolve imported schema)  to be validated).
I am curious couldn't we just test this by default given that all schemas need to be present in "schema/" directory.
we could just load all of them and based on what schema xml reference test that by default. 
Overriding is still fine for all corner cases but by default it shouldn't be needed.