Added documentation to the append method

I know it's stated on the `rootElement` but I feel re-iterating it here is also helpful.

Fixed typo
Merge pull request #2222 from lsdafjklsd/patch-1

Added documentation to the append method
extract Ember.isEmpty into ember-metal

Ember computed.empty should always have identical semantics
Add Ember.computed.none… none + empty have different secants.

Also extract Ember.isNone into ember-metal
Ember.isArray implementation lives in ember-metal, its tests should also

Add test of Ember.isArray against an arrayProxy.
Merge pull request #2228 from stefanpenner/improve_many_small_things

Improve many small things
Stop using extended method

I used `Ember.String.fmt` instead of `String#fmt`.
Because the latter may be undefined.
Fix failing test

This package couldn't be required from test.
Merge pull request #2235 from tricknotes/fix-failing-test

Fix failing test
Better HistoryLocation popstate handling - Fixes #2234
Fix 'rake test[built]' task
Merge pull request #2233 from tricknotes/stop-using-ext-method

Stop using extended method