More progress toward simplifying setup/bootstrap

As part of Issue #206, this implements `mrjob.setup.parse_setup_cmd()`, which parses Hadoop Distributed Cache-style hash paths out of shell commands, leaving the rest of the command completely as-is.

This code is not yet used by `mrjob`; I just figured it's easier to do code reviews piecemeal. Feel free to check out my code and poke things at this function, suggest/write more tests, etc.