The payload-hash-* options are imprecisely named. Clarify their function
as HMAC validations by renaming them. The existing options will continue
to work but are deprecated. Log a warning if the old options are used.
All tests, examples, and documentation are updated.
Fixes#289
Detect if leading character in JSON payload is an array bracket. If
found, decode payload into an interface{} and then save the results into
payload["root"]. References to payload values would need to reference
the leading, "virtual" root node (i.e. "root.0.name").
Fixes#215
