Add support for listing watchers

github/activity_watching.go

@@ -0,0 +1,23 @@
+package github
+
+import "fmt"
+
+// ListWatchers lists watchers of a particular repo.
+//
+// GitHub API Docs: http://developer.github.com/v3/activity/watching/#list-watchers
+func (s *ActivityService) ListWatchers(owner, repo string) ([]User, *Response, error) {
+	url := fmt.Sprintf("repos/%s/%s/subscribers", owner, repo)
+
+	req, err := s.client.NewRequest("GET", url, nil)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	watchers := new([]User)
+	resp, err := s.client.Do(req, watchers)
+	if err != nil {
+		return nil, resp, err
+	}
+
+	return *watchers, resp, err
+}

github/activity_watching_test.go

@@ -0,0 +1,28 @@
+package github
+
+import (
+	"fmt"
+	"net/http"
+	"reflect"
+	"testing"
+)
+
+func TestActivityService_ListWatchers(t *testing.T) {
+	setup()
+	defer teardown()
+
+	mux.HandleFunc("/repos/o/r/subscribers", func(w http.ResponseWriter, r *http.Request) {
+		testMethod(t, r, "GET")
+		fmt.Fprint(w, `[{"id":1}]`)
+	})
+
+	watchers, _, err := client.Activity.ListWatchers("o", "r")
+	if err != nil {
+		t.Errorf("Activity.ListWatchers returned error: %v", err)
+	}
+
+	want := []User{{ID: Int(1)}}
+	if !reflect.DeepEqual(watchers, want) {
+		t.Errorf("Activity.ListWatchers returned %+v, want %+v", watchers, want)
+	}
+}