Also used this opportunity to add a bunch of override's and made some members private.

Author: Reynold Xin <rxin@apache.org>

Closes #340 from rxin/eval and squashes the following commits:

a7c7ca7 [Reynold Xin] Fixed conflicts in merge.
9069de6 [Reynold Xin] Merge branch 'master' into eval
3ccc313 [Reynold Xin] Merge branch 'master' into eval
1a47e10 [Reynold Xin] Renamed apply to eval for generators and added a bunch of override's.
ea061de [Reynold Xin] Rename Expression.apply to eval for better readability.