signature: Use a layered approach for signature verification.

Signature calculation has now moved out from being a package to
top-level as a layered mechanism.

In case of payload calculation with body, go-routines are initiated
to simultaneously write and calculate shasum. Errors are sent
over the writer so that the lower layer removes the temporary files
properly.

typed-errors.go

@@ -23,3 +23,6 @@ var errSyslogNotSupported = errors.New("Syslog logger not supported on windows")
 
 // errInvalidArgument means that input argument is invalid.
 var errInvalidArgument = errors.New("Invalid arguments specified")
+
+// errSignatureMismatch means signature did not match.
+var errSignatureMismatch = errors.New("Signature does not match")