utils.py: adds verify_overlay_src() function

api.py: incorporates the verify_overlay_src() function
api.py: Adds overlay type checking

Checks have been added to determine whether or not to readd a repo
if the overlay type reported from the remotedb is different from the
overlay type reported by the local db.
{argsparser, cli}.py: adds --readd cli option
Merge pull request #5 from twitch153/readd_repos

Adds functionality to re-add a repo based on multiple situations.