Added blocking signal SIGVTALRM during fetch and send

In some scenarios the singal SIGVTALRM can sometimes hit the ruby process with sigreturn EINTR (you can see this using strace). If this signal hits the ruby process during a select() syscall (eg. in _daveReadISOPacket() ) libnodave will return the error -1025 (timeout), which is wrong. Thus we need to block this signal during fetch() and send() calls.