Add users via SSO

[mshwery]

Add single-sign-on (gmail, twitter, facebook?) so users can remember lists (without stacks).

The basic premise is this:

1. Created lists are remembered by client-side storage solutions like localStorage and cookies. These can be wiped at any time, but you'll lose your lists.
2. Alternatively, you can remember lists by signing in with a service like Google, Twitter, etc. This will save your locally stored lists (and load any already saved lists for your account).
3. When authenticated, any lists created will automatically be saved to your account. (might need to add an author_id column to lists if we want controls later on who can edit the list?)
4. Anyone with a link can edit the list. http://getminimalist.com/lists/:uuid would grant you access to add/edit/complete/delete items and rename the list itself.

This helps decouple lists from stacks, so that now sharing a list will only share that one list, as opposed to exposing a stack token. There are probably fewer people who want the functionality of stacks than first imagined. Even we have grown out of it, I would rather have my own set of lists and be able to individually share lists with my wife.

[mshwery]

gmail SSO/oauth has been added now but we should open this up so you could optional sign up with a different email address.

for now: sorry non-gmail users.