ensuring sensu_api_stash payloads are properly formatted

a raw hash was being sent as the request body, which doesn't work.
request body will now be converted to json and then to a string.
Merge pull request #108 from needle-cookbooks/fix-api-stash-payload

ensuring sensu_api_stash payloads are properly formatted
adding definitions for (un)silencing clients and checks
Merge pull request #109 from needle-cookbooks/silence-definitions

adding definitions for (un)silencing clients and checks