[BUGFIX release] Fix #3681.

`Em.arrayComputed` is not presently compatible with lazy CPs. There are a few
issues.

1.  To keep refs up to date, observers stay around as long as the object does.
2.  Those observers can repopulate the cache, but when they do so they fail to
    add dependent keys when the cache was empty.

This commit works around 2. by not removing them in the first place.
Essentially the semantics for `ArrayComputedProperty`s differ from standard
`ComputedProperty`s with regards to laziness.

These observers need to be managed better.  Probably the right thing to do, now
that `recomputeOnce` is no longer necessary, is to:
  1. Fix `Em.computed.sort` to not use `recomputeOnce`;
  2. Fix `Em.arrayComputed` to return a proxy to the cached value;
  3. Teardown array observers when the deps count hits 0.

This will give reduce computed properties semantics much closer to regular
computed properties.  There are two important points to note about this
suggestion:

1.  The cached values would still update synchronously.  This can be improved
    with a lazy array implementation and @wycats's skiplist strategy.
2.  The semantics of an array computed would change such that existing refs
    would only be kept up to date as long as the cache had not been invalidated.
    This is probably fine: the current behaviour is not promised in the
    documentation anywhere and it would be pretty bad practice to keep
    references around rather than `Ember.get`ing them.
[DOC beta] Add documentation to PromiseProxyMixin.
[Bugfix beta] Attribute bindings with a falsy value should always remove the attribute.
[beta] Use defeatureify to strip debug statements.

Recent changes to defeatureify allow stripping of debug messages. This will
allow us to use multi-line debug statements.

To enable this the format of features.json has changed.

See also:
* https://github.com/emberjs/ember-dev/pull/118
* https://github.com/thomasboyt/defeatureify/pull/6
* https://github.com/thomasboyt/defeatureify/pull/8

Hard work done by @thomasboyt & @pangratz.
Merge pull request #4044 from jmonma/master

Attribute bindings with a falsy value should always remove the attribute...
Merge pull request #4038 from rjackson/document_promise_proxy

[DOC beta] Add documentation to PromiseProxyMixin.
[DOC beta] Wrap long Ember.assert messages.
Merge pull request #4045 from rjackson/use_defeatureify_to_strip_debug_statements

[beta] Use defeatureify to strip debug statements.
revert 93a911be370fd6d995badb5e58c6b725a8277ca1

Ember.run.proxy will return as Ember.run.bind
Merge pull request #3707 from hjdivad/fix-3681

[BUGFIX beta] Fix #3681.
[Feature ember-metal-run-bind]
[DOC] Fix invalid syntax for classNames property in link-to helper

Trying to use array syntax throws `TypeError: this.classNames is undefined`
when rendering.
Merge pull request #4055 from praxxis/fix_link-to_classnames

[DOC] Fix invalid syntax for classNames property in link-to helper