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.
overlay.py: adds update() function

To update local database and overlay source urls, this function
has been created to allow the overlay.type functions to update
their source urls in their own methods.

source.py: Adds a stub update() function for overlay types
that don't have their own update() function.