PairOf*String*.java serialization updates

* modify the Pair*String*.java to avoid create String object in comparators.  
* readField, write, compare functions are all updated, 
* implementation iis done by referring to hadoop io Text class. instead of serielize String as UTF, use Text.write(String) to serialize it. so the serialization format is more reliable and consistent with hadoop serialization protocol
* all Pair*String*.java passed the testcases. add one more test to make sure the bytes level comparator was called.