mv version to tulip/version.py

to avoid loading the `tulip` package when importing the version in `setup.py`, because it occurs before dependencies are installed.
This doesn't currently cause problems because `__init__` defines `__all__`, instead of importing specific modules and sub packages.
However it may cause problems in the future, this change intends to address those in advance.

cf http://stackoverflow.com/questions/458550/standard-way-to-embed-version-into-python-package