* Updated TabletRecord class using TabletKey and TabletValue classes.
Changed TabletRecord::getStringValue() into getMutableValue().
* Added TabletKeyList class.
* Added a test case for TabletKeyList class.
* Changed TabletObject interface using TabletKey and TabletValue classes.
* Updated some classes using new TabletKey and TabletValue classes.
* Fixed some methods which uses TabletRecord::getKey().