TaskList is optional on ScheduleActivityTaskDecisionAttributes

http://docs.aws.amazon.com/amazonswf/latest/apireference/API_ScheduleActivityTaskDecisionAttributes.html

Make it a pointer, so it can be nil (and `json:",omitempty"` can do the right thing).

In its current state, calls to the AWS API with empty task listts fail, because an empty TaskList struct is being serialized on the request:

```
2014/10/29 22:13:50 component=FSM name= action=tick at=decide-request-failed error=com.amazon.coral.validate#ValidationException: 1 validation error detected: Value '' at 'decisions.2.member.scheduleActivityTaskDecisionAttributes.taskList.name' failed to satisfy constraint: Member must have length greater than or equal to 1
```