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.
Put HMAC Results Into Local Buffer

HMAC SHA encode functions were putting results into a static array,
which is not thread safe. Declare a local buffer and read the results
into that instead.
Merge pull request #192 from pamaddox/master

Added Check For AWS Cloudwatch