Add APIs to expose the hash key name and range key name of a query builder

Hello @nicks, @giannic, 

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

7d665cfd512a39bedc98a7acf645d57eeb5f02b3 (2014-03-03 00:00:33 -0800)
Generate the default index name based on hash key and range key names

R=@nicks
R=@giannic