Object.assign and Object.mixin

Added Object.assign and Object.mixin.

Implements [19.1.3.1](http://people.mozilla.org/~jorendorff/es6-draft.html#sec-19.1.3.1) and [19.1.3.15](http://people.mozilla.org/~jorendorff/es6-draft.html#sec-19.1.3.15) respectively.

Review URL: https://github.com/google/traceur-compiler/pull/352

Closes #352.