expr: merge on repeated columns raises error
dplyr: transform overwrites repeated columns
expr: fix merge overlapping columns error
expr: eval_str works on unicode literals
replace some instances of str with _strtypes or unicode
Merge pull request #512 from mrocklin/merge-repeated-columns

Merge repeated columns
Merge pull request #515 from mrocklin/str-lingering

Str lingering
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