Fix bugs and improve testing of DictList

Specifically, the following issues were fixed:
 - more robust pickling/unpickling
 - overriding copy and deepcopy now unnecessary (reduce propensity for
   bugs)
 - handle initialization of object without creation of _dict
 - IndexError correctly raised when passed in missing Object
 - pep8 compliance increased
 - docstring cautions against using DictList.remove