[BUGFIX beta] Use better ajax events for counters

Using `ajaxSend` and `ajaxComplete` instead of `ajaxStart` and
`ajaxStop`. See http://api.jquery.com/ajaxSend/ and
http://api.jquery.com/ajaxComplete/ for more information on each one.

It turns out that `ajaxStart` and `ajaxStop` are only called when the
request in question is the only one running. This is problematic for the
`wait()` helper. Changing to events that are called on _every_ request
fixes this problem.
Speed up sendEvent by using direct function calls and `call` instead of `apply` where possible
Merge pull request #4456 from rjackson/revert-removal-of-distros

Revert "Remove unused distros from Assetfile."
Use newly added `apply` and `applyStr` helpers elsewhere. Also minify them so they're inlined.

Breakdown of arguments length while running the Ember test suite:
{0: 444966, 1: 62482, 2: 23473, 3: 14270, 4: 3000, 5: 755}
Fix case where args isn't passed

Scumbag @ebryn didn't rerun all the tests before pushing
Revert "[FEATURE composable-computed-properties] Composable Computed Properties"

This reverts commit f34661cb86d1c359fdc2b1cc1b746c649b97e216.

Conflicts:
	features.json
	packages/ember-metal/lib/array.js
	packages/ember-metal/lib/computed.js
	packages/ember-metal/lib/enumerable_utils.js
	packages/ember-runtime/tests/computed/compose_computed_test.js
Remove unreachable return from blockHelperMissing helper
Merge pull request #4457 from joliss/unreachable-return

Remove unreachable return from blockHelperMissing helper
Fix observer related docs so they show up on the website
Merge pull request #4421 from joefiorini/master

[BUGFIX beta] Use better ajax events for counters
[BUGFIX beta] Move AJAX listeners into Ember.setupForTesting.

Currently, if AJAX is being performed in an initializer the standard
`wait` helper will not be aware of it since the `Test.pendingAjaxRequests` is
initialized to 0 upon `App.injectTestHelpers`. This is AFTER the
application initializers have run.

This changes to allow calling `Ember.setupForTesting` before creating
your application instance (for environments like EAK that create a new
application for each test), and is completely backwards compatible.