check for `wget`, `bagit.py` before running (#4)

[LucasLarson]

bagweb doesn’t first check if both wget and bagit.py are installed before trying to run them: https://github.com/edsu/bagweb/blob/62e4d0625d634bc0ff289f417d3169b1a66f9c7f/bagweb#L3

This pull request to add those verifications – including checks for bagit via both pip and pip3 – will fix #4.