Run dxr-worker.py via sys.executable

When dxr-worker.py gets installed, it can lose its execute bit.  Always run it via sys.executable so we don't have to worry about this (and also makes sure we end up running the correct instance of python to boot).
Merge pull request #97 from mook/worker-run-via-sys-executable

Run dxr-worker.py via sys.executable