Add sort and reverse-sort helpers

[mwpastore]

Here is a first swag at adding (sort) and (reverse-sort) helpers. These simplified versions of the (sort-by) helper allow users to computed-sort arrays of values and not just arrays of objects. They use Ember.compare as the comparator and do not accept any additional arguments.

• (reverse-sort)? (sort-reverse)? Something else?
• Should it just be one (sort) helper with a reverse boolean argument?

[mwpastore]

Hrrm, I didn't touch (previous) or (next) and yet their tests are failing for some reason(s). I experienced the same issue locally with ember try:one ember-release. Any suggestions?