API additions to test query paging with FakeDynamoo

Hello @x-ma, 

Please review the following commits I made in branch 'nick-query'.

7ed419827bb1b179f1e9b041d3ac935c8b9b3a1b (2013-11-18 16:09:05 -0500)
Add an API on the table for setting query limits

fe43fcdf683329f0f626e557cf135a424e9a602a (2013-11-18 16:03:00 -0500)
Fix the query API so that it returns LastEvaluatedKey when there's a limit

R=@x-ma