simplify test suites

the handler should work the same on all platforms, but we do need to
test different versions of chef
move test recipe to its own cookbook, get sensu api from env vars
add optional expiration to stash data
sensu_api_url now defaults to nil, updated recipe logic accordingly
update README and add CHANGELOG
ensure timeout is a fixnum
add test suites for chef 10 and chef 11
update bundler
use a chef 10 friendly error method in test cookbook
add TESTING doc