Session management

Note: I will implement the session checking for the REST api on the unit tests story to avoid doing so much manual testing.

- Users can be created in the web client providing a username and persona authentication
- There is support for registering posting {user:*, pass:*} to /signup. Should we drop it definitely?
- In development mode, you may start a session via any method without registering, if you logged from persona, your username will be test_undefined.
- Admins are created by posting {uid: user-to-add, group:admin } to /setGroup. You must be logged in as an admin to do so.
- The first admin will need to be created manualy, if you don't want to install a mongo shell you could add a snippet like this under the current line 50 from sessions.js (User = db.model('User', userSchema):
```
user = new User({user:*, pass:*, group:'admin'});
user.save(function(){});
```
or with a mongo shell just type: 
```
mongo
> use users-dev
> db.users.insert({user:*, pass:*, group:'admin'})
```
- The web admin panel is under /admin, you can kick users or lock them away from making any changes.