Add marker for futur update of TWI slave mode: reject read/write when busy.
Improving Virtual Wire Interface; adding extended mode with addressing, etc.

1. Fixed RTC::begin so that it may be called from VWI::Receiver::begin() to
avoid problems if forgotten in setup().
2. Fixed iovec_t in Types.h to avoid extensive casting.
3. Added MSLEEP/USLEEP() to Types.h.
4. Added new begin() to VWI to allow node address.
5. Modification of VWI::Receiver::begin() to allow sub-net mask.
6. Added new extended mode header with address, sequence number and
command/type to allow message selection.
7. New VWI::Transmitter send() method with iovec_t parameter. This is the
new primary send function. As this handle both normal and extended mode.
8. Update of VWI examples.