[FEATURE ember-runtime-sortBy] Adds `sortBy` to the Enumerable mixin.

'sortBy' allows for sorting an enumerable by one the values of one or
more properties.

@kselden / @lukemelia
[Improved assert for #3457] provide helpful assertion if needs is specified but no container is present.
Merge pull request #3458 from stefanpenner/improved_needs_assertion

[Improved assert for #3457] provide helpful assertion if needs is specified but no container is present.
Merge pull request #3446 from lukemelia/sort-by

[FEATURE ember-runtime-sortBy] Adds `sortBy` to the Enumerable mixin.