Adding regression test
Make router's render idempotent

This change prevents us from tearing down and recreating a view when
the new view is functionally identical to the old view.

This has important benefits:

 - when you're changing context within a route, the view itself
   remains stable, and the controller has full control over how and
   when to update the data it's presenting to the view
   layer. (Previously, you got a forced re-render immediately whether
   you were ready for it or not.)

 - when you're changing between routes, you can decide to keep the
   prior view in place, and replace it later when you're ready. (Think
   animated transitions between routes.)

This supercedes the proposed changes in PR #2563 (which has already
been reverted).
Clean up pendingDisconnections propertly
Makes partial helper only lookup the deprecated template name if the first try is unsuccessful.
Fix undefined error when promise rejected on startup

Router#targetHandlerInfos is not initialized when a
promise is rejected on startup, causing error
in Router#isActive
Merge pull request #2656 from machty/rejected-promise-app-load-fail

Fix undefined error when promise rejected on startup
Trigger change in fillIn helper in ember testing
Merge pull request #2658 from teddyzeenny/fillin

Trigger change in fillIn helper in ember testing
Merge pull request #2593 from ef4/idempotent-render

Idempotent render
Document and add assertion reflecting that helpers created with registerBoundHelper don't support invocation with Handlebars blocks.

Fixes #2655
Merge pull request #2659 from lukemelia/bound-helper-clarity

Document that registerBoundHelper doesn't support blocks
Merge pull request #2644 from jayphelps/master

Makes partial helper only lookup the deprecated template name if the first try is unsuccessful.
Remove unnecessary reference

`Error.prototype.constructor` returns `Error`.
Remove unnecessary yuidoc markup

The comments begining with `/**` is parsed as yuidoc.
These comments has no yuidoc markup.
Merge pull request #2661 from tricknotes/remove-unnecessary-reference

Remove unnecessary reference
Merge pull request #2662 from tricknotes/remove-unnecessary-yuidoc-comment

Remove unnecessary yuidoc markup
typo
Merge pull request #2665 from gilesbowkett/action_handler_documentation_typo

typo
typo fix.
Merge pull request #2668 from speedmanly/typo-fix

typo fix.