support status query parameter for "/v2/tasks" #271

According to marathon API docs,

GET /v2/tasks Traits: secured
DESCRIPTION
List all running tasks.
QUERY PARAMETERS
status: required, one of (running, staging) string
Filter the list of tasks by status

client.list_tasks() currently not supports this filter.

Also such query parameter is not available for /v2/apps/{app_id}/tasks (which is being used when an appId is passed to client.list_tasks)

Can we have a status filter when appId is None?