add script to upload/download pip cache directories from S3

The user has to do run `pip` or `pip-accel` to actually get packages installed regardless of whether this passes or fails.  I guess we want to make sure this exits with code 0 no matter what happens so that the Jenkins (or maybe eventually local) builds continue on.  Perhaps just a general final try/except here would be good, forwarding any errors to stderr but returning normally?