Option to delete credentials at the end of the execution

[yarivkeinan]

For security reasons, users may want to delete the FB password and maybe even the google token (although it can be revoked from the google side). It would be nice to add another option like '--secure' to remove password at the end of the execution. I am not a python expert so I didn't prepare a pull request (sorry). Should be pretty simple though:

db = shelve.open(FILENAME)
del db[PASSWORD]
# If you need to purge the google token:
# del db[TOKEN]
db.close()