Updated fields.py module.
Updated fields.py module.

* A multiple typed field will insert an empty list into the _fields
  dictionary in __get__ if the field has not been set yet.
  Previously, an empty list was just returned and required the
  receiver to insert the list after modification.
* Refactored unset() function to key off of the _fields dictionary
  rather than the typed_fields property. This should run a bit
  faster as a result (at least with the first run if typed_fields
  hasn't been accessed yet.)

CybOXProject/mixbox