Improve Ember.Array slice implementation

Add support for negative index arguments.
Added documentation for {{partial}}.
Merge pull request #2246 from bestie/negative-array-slice

Improve Ember.Array slice implementation
Remove tab indentation
Remove trailing whitespace
Remove unused variables
Use defined function to remove duplicates
Remove unused argument
Remove old comment

Now it is wrong.
Merge pull request #2255 from tricknotes/remove-unused

Remove unused variables
Merge pull request #2257 from tricknotes/remove-unused-argument

Remove unused argument
Merge pull request #2256 from tricknotes/remove-duplicates

Remove duplicates
Merge pull request #2254 from javierav/indent-whitespace

Remove trailing whitespace and change tab indent by double space indent
Merge pull request #2253 from code0100fun/master

Added documentation for {{partial}}.
remove Ember.metaPath

This seems to be an orphan from the listener refactor in 66bfb53
Add basic implementation of allowed-keys for the {{action}} helper
Merge pull request #2261 from darthdeus/action-allowed-keys

Add basic implementation of allowed-keys for the {{action}} helper
Merge pull request #2260 from igorbernstein/remove-orphaned-code

remove Ember.metaPath
Revert "remove Ember.metaPath" still used by Ember Data

This reverts commit 8b6a5df5cef40231bf2a786eb82530706873a520.
Make Evented#on, #off, and #one chainable