Fix issue #115: Seeking is totally messed up (particularly Byte Seeking)

Tested with Rygel renderer

now we have:

- TIME SEEK OPERATIONS

Seek(offset)
 -> current_position = UPNP GetPositionInfo["RelTime"]
 -> UPNP Seek["REL_TIME", current_position +  offset]

SetPosition(position)
 -> UPNP Seek["REL_TIME", position]

- BYTE SEEK OPERATIONS

ByteSeek(offset)
 -> current_position = UPNP X_DLNA_GetBytePositionInfo["RelByte"]
 -> UPNP Seek["X_DLNA_REL_BYTE", current_position + offset]

SetBytePosition(position)
 -> UPNP Seek["X_DLNA_REL_BYTE", position]