[BUGFIX beta] Ensure namespaces are cleaned up.

Prior to this change, calls to `Ember.Namespace.byName` would still
return the cached class since it was being saved off to
`Ember.Namespace.NAMESPACES_BY_ID` but not cleared.
Merge pull request #4375 from rjackson/ensure-namespaces-cannot-be-looked-up-after-destroy

[BUGFIX beta] Ensure namespaces are cleaned up.
minor doc fixes
directly return default value on get
Merge pull request #4377 from twokul/minor-fixes

minor doc fixes
Merge pull request #4378 from g13013/fix_default

directly return default value on get
[BUGFIX beta] More descriptive error when setting read-only property
Merge pull request #4381 from jayphelps/patch-1

[BUGFIX beta]  More descriptive error when setting read-only property
minor doc UI fixes
Merge pull request #4386 from twokul/minor-doc-fixes

minor doc UI fixes
[DOC beta] view.nearestOfType() is a method not a property
Merge pull request #4393 from MajorBreakfast/patch-4

view.nearestOfType() is a method not a property