avoid import of mrjob library (and its dependencies) to version on setup.py

1. added a version module.
2. moved `mrjob.__version__` to `mrjob.version.__version__`
3. linked `mrjob.__version__` attribute to `mrjob.version.__version__` attribute.
4. added test case for `mrjob.version` module.

not really related but I also added 2 lines on .gitignore to ignore PyCharm .idea directory and .eggs directory.