simplify bootstrapping

This implements the `bootstrap` option, which is meant to replace all the other bootstrap options (except `bootstrap_mrjob`). See #206.

Sorry to leave this hanging for so long!

Next step is to write documentation. I guess it'll be cookbook-style, with an example of how to replace each of the old options (e.g. instead of `--python-archive foo.tar.gz`, do `--setup PYTHONPATH=$PYTHONPATH:foo.tar.gz#`). Would appreciate your input on where to put this in the docs.