Updated note about Firefox and __proto__ restrictions

Correct behavior was implemented in https://bugzilla.mozilla.org/show_bug.cgi?id=1061853.
Update data-es6 as well
Fix JSHint warnings — undeclared vars, semicolons, etc.
Merge pull request #265 from evilpie/patch-3

Updated note about Firefox and __proto__ restrictions
Fixed undeclared var, and added 'use strict' to master.js