Add Number polyfills

This adds the following ES6 Number functions/constants:

  MAX_SAFE_INTEGER
  MIN_SAFE_INTEGER
  EPSILON
  isFinite
  isInteger
  isNaN
  isSafeInteger
The spec is confusing, 
https://people.mozilla.org/~jorendorff/es6-draft.html#sec-number.epsilon 
using base 10 here and 2 else where. I filed a bug.