radiotap and prism header support

[mathieu...@gmail.com]

http://codereview.appspot.com/67125/diff/1/7
File src/common/pcap-writer.h (right):

http://codereview.appspot.com/67125/diff/1/7#newcode26
Line 26: #include "ns3/wifi-mode.h"
oops, you should not include wifi headers from src/common because
src/common is supposed to be dependent on src/core and src/simulator
only. So, either you move the relevant wifi headers in src/common or you
figure out a way to make this code not depend on the wifi types.

A simple way to do this would be to replace the preambl argument with a
bool and the mode argument with an integer containing the data rate in
mbs.

http://codereview.appspot.com/67125