Rework and document OS class

src/main/java/com/sheepit/client/os/Windows.java

@@ -179,11 +179,11 @@ public class Windows extends OS {
 		return false;
 	}
 	
-	@Override public boolean getSupportHighPriority() {
+	@Override public boolean isHighPrioritySupported() {
 		return true;
 	}
 	
-	@Override public boolean checkNiceAvailability() {
+	@Override public boolean isNiceAvailable() {
 		// In windows, nice is not required and therefore we return always true to show the slider in the Settings GUI
 		return true;
 	}