Change GroupMember.AccessLevel to be of type AccessLevel

Fixes #36

groups.go

@@ -201,7 +201,7 @@ type GroupMember struct {
 	Name        string      `json:"name"`
 	State       string      `json:"state"`
 	CreatedAt   time.Time   `json:"created_at"`
-	AccessLevel int       `json:"access_level"`
+	AccessLevel AccessLevel `json:"access_level"`
 }
 
 // ListGroupMembers get a list of group members viewable by the authenticated