Add the scan API to FakeDynamo

Hello @nicks, 

Please review the following commits I made in branch 'xiao-add-scan-to-FakeDynamo'.

f5eabd0aaac392e837bfb4ebca537194cde09fe0 (2013-10-07 17:52:42 -0700)
Add the scan API to FakeDynamo

Check list:


R=@nicks