Create a way to wipe an environment

[Christopher Allan Webber]

It should be possible to do something like:

./bin/gmg wipe_environment mediagoblin.ini

This should then ask the user to confirm (the user should have to type "yes") but should then wipe all:

• sessions
• the database specified in mediagoblin.ini
• the files in the specified public_store and queue_store (assuming they're of BasicFileStore type)