Highly experimental custom sign in

As a title says this is very experimental and I'm not sure if it actually going to make it to the core `gae-init` or in `gae-init-auth` or in a completely separate project.. since many people were asking for it.. I decided to give it a shot.

### Features/Requirements

- [x] Add password salt to the `Config`
- [x] Set password from profile
- [x] Update password from profile
- [x] Sign in using email/password
- [x] Check for unique email
- [x] Check for conflicts when duplicate email during sign in
- [ ] Send conflict notification to the admins
- [ ] Sign up with email/password
- [ ] Reset/Forgot password

#### Few things about this _experiment_:

- It's not finished
- There are no repeat passwords fields.. the latest trend says that they are annoying.. :)
- Bunch of things are subject to change and I'm open to any suggestions
- Since the visuals were never the top priority for `gae-init` I don't want to start creating bunch of custom css/js (stuff like transitions, client side validations, etc)
- The server side should be robust and bulletproof