Minimal switching tentatively working

No tests, not all edge cases covered, must specify an exact tag, don't handle dependencies, but a good start!
Subprocess tweaks (thanks Jeremy)
Merge pull request #602 from kastman/ENH_VersionChooser

Minimal Version Chooser
RF: reduce os.chdir calls, initial commit

- sometimes get a test failure with os.getcwd() failing, seems like maybe an earlier chdir left
  things hanging
- info.py not changed

RF/BF: remove chdir from info._getHashGitHead

- BF = enable default directory to work

RF+BF: always use absolute path for data file in Builder scripts

- needed after removing os.chdir prior to running a script from builder.py / experiment.py
  IMHO always working with abs paths is an improvement
- allow user to specify a relative or absolute path is settings:
  use the users's explicit abs path if its abosolute, otherwise use the abspath of the script's dir
- BF: handle case of user deletes data file name

RF: smoother handling of missing datafile name in settings

RF: ensure expected cwd from inside builder scripts

FF: info.py getting git-hash of psychopy

TEST: new test of info.py

RF: remove commented-out code having os.chdir