add scipy to requirements.txt and to setup.py
use platform-agnostic downloader

wget is not available by default on Mac; curl is not available by
default on ubuntu.  Use urllib to retrieve rather than rely on system
call
perform requirements check at start of data download
move rebin() to helpers.py
version bumps for requirements.txt
setup.py reads requirements.txt
add create-metaseq-test-environment.sh
prompt number changes in example sessions