remaining todos

- write some mixed sugar tests
  - filter then sort
  - map then union
- non-array reduce cases, eg min, max
- Em.arrayComputed.map should allow multiple dependent keys as in

    things: [{a:1, b:1, c:1}, {a:2, b:2, c:2}, {a:3, b:3, c:3}]
    evens: Em.arrayComputed.map(
      function(i){ return i.a % 2 == 0 || i.b % 2 == 0},
      'things.@each.a', 'things.@each.b')

- refactor: split core from sugar in array_computed.js