Fix for #12 Native defaultdict for 2.5+

[reachtarunhere]

Problem
The following is the fix as per the suggestion in #12 which suggests
The collections.defaultdict class http://docs.python.org/library/collections.html#collections.defaultdict
could be used for Python 2.5+ rather than the method in utils.py

Solution:
The DefaultDict function implementation does the above and also maintains backward compatibility taking into account the fact that the native implementation in 2.5+ requires a callable for setting default values and the original implementation made use of non callables like 0 or []. The implementation allows using both a callable or the previous method.

[reachtarunhere]

I am closing this since I don't feel that this is relevant anymore given we are ok with breaking compatibility for the main branch. Though it might be a good idea to add this to the legacy branch.