Added Check From AWS Cloudwatch

Added ability to pull performance metrics from AWS Cloudwatch. Check
takes a namespace and a list of metrics and dimensions and queries
the API to pull these metrics.

Added ability to create HMAC SHA256 encoding into the backend to
sign messages to the API.

Four changes:

* Change "metrics" field to "cloudwatch_metrics".

* If no period is specified, assume a 5-minute period for the purposes
  of pulling data.

* Only append the statistic value to the metric name if it is not "Average"

* If no value is found for a metric, still return the metric - just assign
  it a null value.