Added a new setting to allow multiple Django users

[cesarblancg]

I'm using your Django app, and my website will have many users, and I wish every user has their Deviantart token, so I modified it.

I added a new setting option:
DEVIANTART_ALLOW_USER = True/False

If it's set to True, every Django account will have their token, and won't share it (this mode will display an error if you use it without authenticate).

If it's set to False, your website (Anonymous users and users authenticated) will have the same token.

[bloodywing]

I started to review your changes. But it looks like your django migrations are missing for this.
Besides that, I would like you to invite as collaborator for this django app, I'm not actively coding on this django app but it sure needs some fresh ideas.