Add EmberCPM.Macros.sumBy.

Operates on an array of objects, and sums a given property on the
object. The implementation is essentially the same as
`Ember.computed.sum` with a few changes (to get the value of a given
property instead of just adding each item).
Merge pull request #18 from rjackson/add-sumBy

Add EmberCPM.Macros.sumBy.