Object level permissions

### Includes:

* [django-guardian](http://django-guardian.readthedocs.org/) for object-level permissions
* new community permissions
* removal of creating generic groups, now for every community 4 new groups are created
* catching of `post_save` and `post_delete` signals to create community groups and assign permissions to each group
* unittests

### How it works:

1. Create a new community object called “Foo”
2. On saving new community “Foo” create groups:
    * “Foo Content Contributor” 
    * “Foo Content Manager”
    * “Foo User and Content Manager” 
    * “Foo Community Admin”
3. For each newly created group assign permissions - assign_perm('some_permission', group_name, community_foo)
4. If you want to give "Content Contributor" permissions to this community to a specific user, make user member of the “Content Contributor for Foo”
5. In views check if user has permissions inside functions or using decorators

### Commits explained

* "create Community object-level groups" 3bbe798 - adds new permissions to Community model, which reflect some actions/permissions on other models. When a new community is created, 4 community related groups are created.
* "create Generic permissions group on post_migrate" 77b17b4 - creates a single generic group after all models get created. It contains permissions that a user from any other group will need - manipulating tags, resourcetype and cms plugins permissions.
* "grant access to parent community admins " 1e0175f - implements the feature that admins of parent communities have Community Admin permissions for children communities.
* "manage community admin group on updating Community profile" 83d0721 - admin of a Community automatically becomes a member of group "Community Admin for <community.name>"

This is how it looks like in the admin dashboard
![screenshot from 2014-07-15 13 13 36](https://cloud.githubusercontent.com/assets/2039122/3590276/1d1340b4-0c57-11e4-9e66-21d9b180ab4a.png)

More details and discussions here - https://docs.google.com/document/d/14Xw-Xb82tXDdIki7-ZgppoUbwQ6DpmBP9fEiBm8zvLQ