Added RawVal object that ca be assigned to any packet field and will inserted...

Added RawVal object that ca be assigned to any packet field and will inserted as-is in the packet string

ex:
>>> a=IP(len=RawVal("####"))
>>> str(a)
'F\x00####\x00\x01\x00\x005\xb5\x00\x00\x7f\x00\x00\x01\x7f\x00\x00\x01\x00\x00'