Add customizable Logger interface + info,error and debugs (#14)

* Add customizable Logger interface + info,error and debugs
* update travis.yml to atest golang versions

logger.go

@@ -0,0 +1,44 @@
+package webhooks
+
+import "log"
+
+// DefaultLog contains the default logger for webhooks, and prints only info and error messages by default
+// for debugs override DefaultLog or see NewLogger for creating one without debugs.
+var DefaultLog Logger = new(logger)
+
+// Logger allows for customizable logging
+type Logger interface {
+	// Info prints basic information.
+	Info(string)
+	// Error prints error information.
+	Error(string)
+	// Debug prints information usefull for debugging.
+	Debug(string)
+}
+
+// NewLogger returns a new logger for use.
+func NewLogger(debug bool) Logger {
+	return &logger{PrintDebugs: debug}
+}
+
+type logger struct {
+	PrintDebugs bool
+}
+
+// Info prints basic information.
+func (l *logger) Info(msg string) {
+	log.Println("INFO:", msg)
+}
+
+// v prints error information.
+func (l *logger) Error(msg string) {
+	log.Println("ERROR:", msg)
+}
+
+// Debug prints information usefull for debugging.
+func (l *logger) Debug(msg string) {
+	if !l.PrintDebugs {
+		return
+	}
+	log.Println("DEBUG:", msg)
+}