Bug #225: Use user supplied path to Python in preference to sys.executable

This allows custom scripts to wrap python and be used by PyDev. In these
scripts, stuff like LD_LIBRARY_PATH can be used. An example is CCTBX
python: http://cctbx.sourceforge.net/
Merge branch 'python_cctbx_support' of https://github.com/jonahkichwacoders/Pydev into jonahkichwacoders-python_cctbx_support
Merge branch 'jonahkichwacoders-python_cctbx_support' into development