Use widening with addition and subtraction

The original design decision was made in part to anticipate changes
Julia 0.4. Unfortunately, this runs into major trouble with
reductions. Since we need a package that works on 0.3, these changes
seem necessary. Once someone implements widening in the reduction functions
for 0.4, this change can probably be reverted.
Merge pull request #12 from timholy/pull-request/854d8a8d

Use widening with addition and subtraction