Issue #488 Implementation of teams

Hi,

I've implemented teams for pybossa:

Steps:
  Create new Tables: Team and User2Team
  New Item in Settings User and logical file for Teams (team.py)
  Create Templates html for teams (New folder templates/team)
  Unit Test for these implementations (test_team.py)

In this issue I have not included stats for these new teams. I will create a new issue for this task.

I hope you like it all these changes