Added eventsMutex and wrap all events calls with it.

Due to some "golangish" code this library have possibility to
run into data race when application is working with callbacks.
This commit adds eventsMutex (which is a sync.Mutex), removed
all "golangish" ifs-map reads, and wrap events map read with
sync.Mutex to avoid data races.

irc_struct.go

@@ -39,6 +39,7 @@ type Connection struct {
 	user        string
 	registered  bool
 	events      map[string]map[int]func(*Event)
+	eventsMutex sync.Mutex
 
 	QuitMessage string
 	lastMessage time.Time