expr: Map.label short-circuits old map
Merge branch 'master' of github.com:ContinuumIO/blaze
expr: Expr.project used to simplify graphs

Classes like Column, Project, Merge can all project more intelligently
than by just wrapping around another Projection.
expr: py3 import unicode from compatibility
expr: merge.project(list-of-columns) fixed and tested
Merge pull request #516 from mrocklin/expr-project

Small optimizations to expressions