mark Promise as a constructor so it can be used in type annotations

Hello @x-ma, 

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

a1af31ad6e4414712ca1a36b86d02f3756595c33 (2014-03-05 19:59:03 -0500)
mark Promise as a constructor so it can be used in type annotations

R=@x-ma