Fixed project model import in `bb_tasks` models module.
Fixed circular import in `bb_tasks`.
`tasks` factory models fixed.
Set up `locmem` backend for email testing.
`task_id` field added to `TaskMember` to relate members with tasks.
Fixed `task_id` field to be nullable.
Fixed signal handlers for `bb_tasks`.
Tasks mailing signal handlers refactored.
Mailing unit tests fixed.
Model test removed (useless because it is already tested by the factory
models).
Merge pull request #246 from jose-lpa/refaktor

`bb_tasks` app fixes
Renamed templates directory for `contact` application.
Fixed includes in `bb_organizations/base.hbs` template.
Compression deactivated in testing settings.
Core API URLs explicitly specified.
Cleaned up API unit tests in `bb_tasks`.
Merge pull request #249 from jose-lpa/refaktor

Some more fixes to the `bb_tasks` app.
Fixed `bb_tasks` serializers.
Merge pull request #252 from jose-lpa/refaktor

Fixed `bb_tasks` serializers.
Merge branch 'refaktor' into release/bb-refaktor