Fixed #109. Users collection has a status field.

We now have an option to either mark a user active via direct signup, or
mark a user as invited via invitation. Bad users can be marked as
'banned'. When a user is marked as invited, the user is given a url
(front-end) to login. Upon the first login, the status will change to
'active'. Ticket #111 will check user's status and prevent banned users
(and non-active) users from accessing Minion.
Added SMTP call so POSTing to /users can send invitation. See #110.
Update README.md to include postfix as dependency.
Update setup.py to include minion-db-init.
Fixed #109. Users collection has a status field.

We now have an option to either mark a user active via direct signup, or
mark a user as invited via invitation. Bad users can be marked as
'banned'. When a user is marked as invited, the user is given a url
(front-end) to login. Upon the first login, the status will change to
'active'. Ticket #111 will check user's status and prevent banned users
(and non-active) users from accessing Minion.
Added SMTP call so POSTing to /users can send invitation. See #110.
Update README.md to include postfix as dependency.
Update setup.py to include minion-db-init.
Added POST /invites. See #114.
Added GET /invites with basic filters. See #114.
Merge branch 'issue109110' of https://github.com/yeukhon/minion-backend into issue109110

Conflicts:
	minion/backend/api.py
Added GET /invites/<id>. See #114
Add POST /invites/<id>/control. See #114
Remove send-email from /users. We moved that to /invites. See #114.
Added DELETE /invites/<id>. See #114.
Fixed delete /invites/<id> doesn't delete user.
Fixes #115 Replace the Twisted code in the plugin-worker with plain python stdlib code
Add status and max_time_allowed for invites collection.
Changes to make the backend more production ready
Merge pull request #113 from yeukhon/issue109110

Add status to user field, add simple SMTP