Move "realm" attribute to be on <http> element rather than <http-basic>.

This faciltiates reuse with other mechanisms (like Digest) whilst also 
moving towards the <http-auto-configure> element (which benefits from 
having shared configuration in <http> as opposed to mechanism-specific 
elements).

samples/tutorial/src/main/webapp/WEB-INF/applicationContext-security-ns.xml

@@ -19,7 +19,7 @@
 
         <form-login />
         <anonymous />
-        <http-basic realm="SpringSecurityTutorialApp"  />
+        <http-basic />
         <logout />
         <concurrent-session-control maxSessions="1" exceptionIfMaximumExceeded="true"/>