Move st2client setup from pbr to setuptools

Use default setuptools instead of pbr for more control on versioning. Remove pbr from requirements.
Allow st2client setup to be run from other dir

Get realpath to setup.py and then change to the directory where setup.py is located.
Merge pull request #801 from StackStorm/STORM-716/st2-version

Move st2client setup from pbr to setuptools