[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.
[beta] Replaces anonynoums functions with named ones
[DOC beta] Assert if template for {{render}} not found.

Added an assertion if the template name provided to `{{render}}` was not
found.

Two tests were previously not working properly, and have been fixed. They registered
templates as `"blog/post"`, which is never looked up with `{{render}}` (as any
slashes in the name are translated to a period before this lookup
happens).

Resolves #3926.
[BUGFIX beta] preserve base URL when using history location for routing
Assert if {{render}} is called with an unquoted name.

Eventually, `{{render}}` should be similar to `{{partial}}` where an
unquoted templateName is used to do bound parameter lookup.

This PR updates the tests to use quotes, and adds an assertion if an
unquoted param was provided.
add Ember.computed.sum
unbound helper supports bound helper static strings
Ember.run.proxy + Ember.run.method

run-loop aware helpers for more ergonomic integration
with 3rd party asynchronous callbacks.
[DOC beta] Ember.View documentation cleanup.
[DOC beta] Several doc fixes
[DOC beta] Cleanup Ember.TrackedArray docs.
Label promises for debugging
[beta] Update handlebars to 1.2.1.
Merge pull request #3927 from rjackson/warn_on_missing_template

[DOC beta] Assert if template for {{render}} not found.
Merge pull request #4002 from twokul/runloop-doc-fixes

[DOC beta] Several doc fixes
Merge pull request #4003 from rjackson/cleanup_tracked_array

[DOC beta] Cleanup Ember.TrackedArray docs.
Merge pull request #4001 from rjackson/view_doc_cleanup

[DOC beta] Ember.View documentation cleanup.
Merge pull request #3944 from rjackson/cleanup_unquoted_render_usage

Assert if {{render}} is called with an unquoted name.
Merge pull request #3946 from teddyzeenny/new-promise-labels

Label Promises
Merge pull request #3999 from rjackson/update_handlebars

[beta] Update handlebars to 1.2.1.