RESTEASY-180 Adjusted scope for some dependencies in poms.

SLF4J-Simple was appearing as a runtime dependency, but it should not because it is a "logging implementation" and that choice should be the responsibility of the application (not a library).
Upgraded SLF4J and added a property to manage the version (like some other dependencies).
JUnit is a test framework, so it should have test scope.  Alas, while changing the default, there are some example code that use JUnit directly, so had to explicitly mention compile scope.  (I suspect that code could be moved to the test tree in the future to be more consistent with convention.)
Servlet API is now declared with provided scope by default, since its implementation is typically provided by the container (JBoss, Tomcat, Jetty).