Fixed small bugs on intergas order form.

Added error message when a user tries to add an intergas order between two or more GAS that do not have a pact with the supplier tha refers to the order pact.
Added integrity and type checks on the GAS model clean method.
Starting to develop new GUI.

This GUI will be useful to ease some operation that do not need to be done often (like GAS creation). This GUI could also be improved in the future to cover more operations.
Updated requirements: django-notification.

Now it refers to feroda fork of bosner/django-notification
Fixed small bug on intergas form.

Intergas form fieldset need to be a `deepcopy` of super form fieldset.
First implementation of a basic tag to render notifications.

Used into the list_notifications view template.