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/timings_plot_reporter.go

@@ -20,8 +20,8 @@ func NewTimingsPlotReporter() *TimingsPlotReporter {
 	return &TimingsPlotReporter{results: list.New()}
 }
 
-// add inserts response to be used in the report, sorted by timestamp.
-func (r *TimingsPlotReporter) add(res *Result) {
+// Add inserts response to be used in the report, sorted by timestamp.
+func (r *TimingsPlotReporter) Add(res *Result) {
 	// Empty list
 	if r.results.Len() == 0 {
 		r.results.PushFront(res)