[BUGFIX beta] reduceComputed handle out-of-range index.

f3aa4a2d0854395e1f8158a74d1810c290a75139 added support for `replace`s on
dependent arrays with out of range indexes, similar to what `[].splice`
supports.  However, it had two errors that are corrected in this commit.

1.  An off by one error when determining the normalized index for removal
2.  It did not limit the removed count by the number of items that could possibly
    be removed.

Change `normalizeIndex` to the more natural signature of `index, length, count`.
This has no effect on any public API.

Big thanks to @Cyril-sf for pairing on this fix.

Fix #3586 (again).
[FEATURE reduceComputed-non-array-dependencies] reduceComputed supports non-array dependencies.

`ReduceComputedProperty`s want 3 kinds of dependencies.

1. non-array dependencies
2. array dependencies with normal (total invalidation) CP semantics
3. array dependencies with one-at-a-time semantics

It was previously only possible to create `reduceComputed`s with 3.  Macros
could be written that would manually handle 1. and 2.  For example,
`Ember.computed.sort` does this.

This commit makes possible all three cases.

```js
Ember.Object.extend({
  nonArray: 'string',
  totallyInvalidatingArray: [],
  oneAtATimeArray: [],

  computed: Ember.reduceComputed( 'nonArray',
                                  'totallyInvalidatingArray.[]',
                                  'oneAtATimeArray', {
    addedItem: function () { /* ... */ },
    removedItem: function () { /* ... */ }
  });
})
```

Arrays whose individual mutations should completely invalidate a
`reduceComputed` are specified by adding the `.[]` suffix to the dependent key.
Note that the items for this array are never passed to the `addedItem` and
`removedItem` callbacks: they should be treated much like non-array properties.

A straightforward motivating example is `Ember.computed.sort` - the items to be
sorted should be handled one-at-a-time, but the properties to sort by should
require a complete resort.

Fix #3600.
Merge pull request #3627 from hjdivad/fix-3586-maybe-for-reals-this-time

[BUGFIX beta] reduceComputed handle out-of-range index.
Merge pull request #3614 from hjdivad/reduce-computed-nonarray-dependencies

[BUGFIX beta] reduceComputed supports non-array dependencies.
[FEATURE propertyBraceExpansion] Better code stripping