added task_link to asset model
added new field task_link to DC & BO (search and edit device)
fixed BaseEditAssetForm.clean() (added super(..) call)
splited  EditDeviceForm to BackOfficeEditDeviceForm, DataCenterEditDeviceForm,
added validation to task_link field
used dependency feat. on task_link field
removed blank line before pass
task_link model max_length increased to 2048
sorted imports in forms.py
removed a useless todo
Merge branch 'develop' into task-link

Conflicts:
	src/ralph_assets/views.py
Merge branch 'develop' into task-link
Merge pull request #115 from xliiv/task-link

Task link