Improved `merge_dict` and included few docstrings

Python dictionaries have `update` method. That would be far more efficient than converting the `dict.items` to list and creating a `dict` at the end.
Cleaned up few lines of code

I am not very comfortable with the usage of `six`, which is not a built in module.
Merge pull request #31 from thefourtheye/patch-1

Improved util.merge_dict and included few docstrings