Object.mixin() has been removed from the es6 spec.

Removed as of the Jan 20, 2014 es6 draft.

Closes #192.
Merge pull request #193 from cscott/gh-192

Object.mixin() has been removed from the es6 spec.
Update docs after removing Object.mixin()

Follows up on #192 and #193.
Merge pull request #194 from arlolra/mixin

Update docs after removing Object.mixin()