Add support for TestRail timestamp type

Convert a UNIX timestamp into a native time.Time

timestamp.go

@@ -0,0 +1,20 @@
+package testrail
+
+import (
+	"encoding/json"
+	"time"
+)
+
+// UNIX timestamp
+type timestamp struct {
+	time.Time
+}
+
+func (ts *timestamp) UnmarshalJSON(data []byte) error {
+	var seconds int64
+	if err := json.Unmarshal(data, &seconds); err != nil {
+		return err
+	}
+	ts.Time = time.Unix(seconds, 0)
+	return nil
+}