add computed.alias
Deprecate Ember.alias in favour of Ember.aliasMethod

https://github.com/emberjs/ember.js/issues/1725
alias and aliasMethod differ.

alias will continue support for computedProperties, but aliasMethod will
not.
Ember.computed.alias supports setting.
fixup typo
Ember.computed.alias tests
Improve Docs of Ember.alias and Ember.aliasMethod
Refine Ember.aliasMethod tests
Merge pull request #1728 from stefanpenner/alias_updates

WIP Implemenation of #1725