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/text_reporter.go

@@ -69,8 +69,8 @@ func (r *TextReporter) Report(out io.Writer) error {
 	return w.Flush()
 }
 
-// add adds a response to be used in the report
+// Add adds a response to be used in the report
 // Order of arrival is not relevant for this reporter
-func (r *TextReporter) add(res *Result) {
+func (r *TextReporter) Add(res *Result) {
 	r.results = append(r.results, res)
 }