From: http://ejohn.org/blog/ecmascript-5-objects-and-properties/
Writable: If false, the value of the property can not be changed.
Configurable: If false, any attempts to delete the property or change its attributes (Writable, Configurable, or Enumerable) will fail.
Enumerable: If true, the property will be iterated over when a user does for (var prop in obj){} (or similar).