Avoid circular dependencies

The current setup.py imports __version__ from datetimes/__init__.py, which depends on pytz (which setup.py will install). That makes circular dependencies. The user will have to manually install pytz to install datetimes.

Reference: 
1. [How can I get the version defined in setup.py (setuptools) in my package?](http://stackoverflow.com/questions/2058802/how-can-i-get-the-version-defined-in-setup-py-setuptools-in-my-package)
2. [Automatic version number both in setup.py (setuptools) AND source code?](http://stackoverflow.com/questions/6786555/automatic-version-number-both-in-setup-py-setuptools-and-source-code)