Optional merge strategies

[kbroughton]

Add the ability to specify how merges are made on lists, sets or other objects.

Fixes optional merge strategies for lists/sets #10

[kbroughton]

The test isn't passing, but the value looks correct. Please advise.

FAIL: test_update_with_combine_policies (tests.test_nested_dict.Test_nested_dict_methods)

Test update with combine_policies

Traceback (most recent call last):
File "/Users/kestenbroughton/projects/nested-dict/tests/test_nested_dict.py", line 257, in test_update_with_combine_policies
self.assertEqual(nd1.to_dict, {'a':1,'f':[1,3,3,2]})
AssertionError: <bound method nested_dict.to_dict of defaultdict(<class 'nested_dict.implementation.nested_dict'>, {'a': 1, 'f': [1, 3, 3, 2]})> != {'a': 1, 'f': [1, 3, 3, 2]}

[kbroughton]

tests are passing now. It fails after during doc build.