Add Number polyfills

This adds the following ES6 Number functions/constants:

  MAX_SAFE_INTEGER
  MIN_SAFE_INTEGER
  EPSILON
  isFinite
  isInteger
  isNaN
  isSafeInteger
Did you intend to allow conversions? Since we are in the business of coercions here, I think a comment would clarify that the conversion rules were being applied. (We don't know the type of `number`).