Incorporated patches and bug fixes from community (tweaked slightly).

Incorporated patches and bug fixes from community (tweaked slightly).
  - Added support for getting RSSI of packets (Razvan Musaliou-E. from
    JHU). Added interface TossimPacket for getting this value without
    needing to know metadata layout. This required changing
    TossimActiveMessageP to TossimActiveMessageC as applications may need
    to wire to it.
  - Tweaked noise simulation so output values are not quantized.
    Tal Rusak from Cornell observed that quantization made borderline
    links behave badly.
  - Modified apps/tests/TestSimComm so that it prints out the RSSI
    of packets.