Add an Ordering for NullWritable to make the compiler generate same byte codes for RDD

`NullWritable` is a `Comparable` rather than `Comparable[NullWritable]` in Hadoop 1.+, so the compiler cannot find an implicit Ordering for it. It will generate different anonymous classes for `saveAsTextFile` in Hadoop 1.+ and Hadoop 2.+. Therefore, here we provide an Ordering for NullWritable so that the compiler will generate same codes.

I used the following commands to confirm the generated byte codes are some.
```
mvn -Dhadoop.version=1.2.1 -DskipTests clean package -pl core -am
javap -private -c -classpath core/target/scala-2.10/classes org.apache.spark.rdd.RDD > ~/hadoop1.txt

mvn -Pyarn -Phadoop-2.2 -Dhadoop.version=2.2.0 -DskipTests clean package -pl core -am
javap -private -c -classpath core/target/scala-2.10/classes org.apache.spark.rdd.RDD > ~/hadoop2.txt

diff ~/hadoop1.txt ~/hadoop2.txt
```

However, it's inevitable that generating different codes for the classes which call methods of `org.apache.hadoop.mapreduce.TaskAttemptContext`. `TaskAttemptContext` is a class in Hadoop 1.+, and calling its method will use `invokevirtual`, while it's an interface in Hadoop 2.+, and will use `invokeinterface`.