JAVA-428 - Support new ReadPreference semantics, deprecate SlaveOK

* Added toStrings for clarification to Concrete ReadPreference instances

Author: Brendan W. McAdams

src/main/com/mongodb/ReadPreference.java

@@ -18,10 +18,18 @@
 public class ReadPreference {
     public static class PrimaryReadPreference extends ReadPreference {
         private PrimaryReadPreference() {}
+        @Override
+        public String toString(){
+           return "ReadPreference.PRIMARY" ;
+        }
     }
 
     public static class SecondaryReadPreference extends ReadPreference {
         private SecondaryReadPreference() {}
+        @Override
+        public String toString(){
+            return "ReadPreference.SECONDARY";
+        }
     }
 
     public static class TaggedReadPreference extends ReadPreference {
@@ -37,6 +45,11 @@ public DBObject getTags(){
             return _tags;
         }
 
+        @Override
+        public String toString(){
+            return getTags().toString();
+        }
+
         private final DBObject _tags;
 
     }