added autoscaling describeScalingActivities support

The ID here is a guid (maybe I'll get un-annoyed by this one day):
```
{
activityId: "c1111e1e-1b11-11fe-1da1-fd11a111e1cf",
autoScalingGroupName: "asgname",
cause: "At 2013-11-11T11:11:11Z a monitor alarm asg-alarm-name in state ALARM triggered policy asg-policy-up changing the desired capacity from 1 to 2. At 2013-11-11T11:11:11Z an instance was started in response to a difference between desired and actual capacity, increasing the capacity from 1 to 2.",
class: "com.amazonaws.services.autoscaling.model.Activity",
description: "Launching a new EC2 instance: i-abcdef78",
details: "{"InvokingAlarms":[{"Region":"US - N. Virginia","OldStateValue":"OK","AWSAccountId":"blahblah","AlarmName":"asg-alarm-name","AlarmDescription":null,"StateChangeTime":1385746272474,"NewStateReason":"Threshold Crossed: 1 datapoint (85.679) was greater than or equal to the threshold (85.0).","NewStateValue":"ALARM","Trigger":{"Period":300,"Statistic":"AVERAGE","MetricName":"CPUUtilization","Threshold":85,"EvaluationPeriods":1,"Dimensions":[{"name":"AutoScalingGroupName","value":"asgname"}],"Namespace":"AWS/EC2","ComparisonOperator":"GreaterThanOrEqualToThreshold","Unit":null}}],"Availability Zone":"us-east-1d"}",
endTime: 1385746425000,
progress: 100,
startTime: 1385746301644,
statusCode: "Successful",
statusMessage: null
}
```
Also, Details looks like it's being mapped as a string which I'd like to parse out to seperate fields but I'm not sure how dynamic that data is yet or if it's usable as is.