Add AWS cloudwatch datasource

Adds a new [AWS CloudWatch](http://docs.aws.amazon.com/AmazonCloudWatch/latest/APIReference/Welcome.html) datasource for metrics, using the [JS aws sdk](https://www.npmjs.org/package/aws-sdk).
Would it be possible to source the jdk at the end, and only if we've identified that the user intends to use it? e.g. add a `var usingCloudWatch = true;` here and filtering on that condition later on for the script source?