Fix: only write data back to nbd when responding to a read request, fixes #28

Add: debug print of packet
Also add checking of magic field in packets from nbd.
Add: debug print function of packets
Fix: making sure to read all bytes from connection

When reading from a connection, the Read(b []byte) function doesn't always read up to len(b). To fix this, a new helper function is impemented.
Fix: lots of fixes, Add: uses a map for mapping nbd handler with ivbs sequence

New helper functions to make sure bytes are read and written to connections and files.
Change: using net.TCPConn instead of net.Conn