Enable login System

## Need
```gherkin
In order to make my activity in ROSEdu public
As a ROSEdu comunity member
I want to be able to sign up and login
```
## Deliverables

A login / sign-up system that doesn't allow random signups.

## Solution

- Allow anybody to login and signup
- Add a field that allows only some of the trusted users to add their projects.

#### TODO
- [x] Use django.contrib.auth users in the Person model
- [x] Add a login page
- [x] Make profile_set work for an user only if he is logged in
- [x] Make the template change when an user is logged in
  - Show a link to the user and a link to logout
  - Show the edit link only on self profile page
- [x] Add change password to profile set
- [x] Add a signup page
- [x] Add a basic authorization system
- [ ] Modify initial fixture

## Notes
Having a free for anybody login system would allow anybody to add himself in all the projects and would make the data irrelevant.

Signup page will need the following information displayed:
- Name
- Email
- Password
- Link to use gravatar