+ install nosql-encoding package only to use(no need to install encod…

…ing-iterator now).

* database developer should not force inherited from the Encoding class. Let the user decide whether to use the Encoding feature please.

* [bug] mGetSync/mGet return values array, but not decode value.

* [bug] forget callback argument to approximateSizeAsync

* [bug] typo on approximateSize

* [bug] batch should ignore error argumemts

* [bug] a little typo on mGetAsync.

* [bug] many bugs fixed and test added.