Feature/bb refaktor pygraphviz

TO ADD IN THE WIKI:
[https://code.djangoproject.com/wiki/DjangoGraphviz]
[http://django-extensions.readthedocs.org/en/latest/graph_models.html]

USAGE:

commands:

# Create a dot file of the whole project:
$ python manage.py graph_models -a > file_name.dot

# Create a PNG image file with application grouping
$ python manage.py graph_models -a -g -o file_name.png

# Create a dot file for only the 'accounts' and 'projects' applications of your project
$ python manage.py graph_models accounts projects > file_name.dot

# Convert dot file into png file
$ dot -Tpng accounts_projects.dot > accounts_projects.png