Add command option --executable

I have added a command option `--executable` that can be used to pass the path of a Python-executable to the commands `worker --detach` and `multi`. This Python-executable will then be used instead of `sys.executable`. This patch fixes the problem of running the Celery daemon inside of Buildout, as described by [issue 364](https://github.com/celery/celery/issues/364), especially [the last comment by bnkr](https://github.com/celery/celery/issues/364#issuecomment-16454187).

I haven't written any tests, because I have no clue how I should test this. Sorry for that.