Decouple Reporters from Attack function

This changeset breaks the API of Attack in order to decouple Reporters
and the Attack function. Attack now returns a slice with
non-deterministic order of Results which one can use on the calling code
with or without Reporters, hence making it much more useful on a library
usage setting.
These developments could be of interest to issue #11 which was closed in
the past.

lib/reporter.go

@@ -7,5 +7,5 @@ import (
 // Reporter represents any reporter of the results of the test
 type Reporter interface {
 	Report(io.Writer) error
-	add(res *Result)
+	Add(res *Result)
 }