Fix the consumed capacity of batchGetItems() in FakeDynamo

Hello @nicks, @giannic, 

Please review the following commits I made in branch 'xiao-fix-capaciy-batchget'.

42eee2728684d269893108bdf12f407889a2d8ac (2014-02-26 15:04:56 -0800)
Fix the consumed capacity of batchGetItems() in FakeDynamo

R=@nicks
R=@giannic