Make FakeDynamo return the right capacity information

Hello @nicks, @dpup, 

Please review the following commits I made in branch 'xiao-FakeDynamo-capacity'.

30ee0fdea589a2568e5495a2643c628ecd189555 (2014-01-31 15:09:40 -0800)
Make FakeDynamo return the right capacity information
For more information - http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_Operations.html

R=@nicks
R=@dpup