PEP-8 fixes. Refs #371
Migration and Model added. Refs #371

This commit adds the migration, updates the model and also the app presenter and tasks to participate.
Update and Create methods updated. Refs #371
Unit Tests. Refs #371
Unit Tests. Closes #371
Updated docs. Refs #371
Merge pull request #420 from PyBossa/issue-371-force-signin

Issue 371 force signin