run with time command
updated .gitignore
OutputFileStream for logging
DifsWifiMacTest as simulation class
mobility settings were moved to ConstantMobilityNodeContainer class
MAC and PHY preparations were moved to WifiNodeContainer class
getNetDeviceContainer and getDevice in ConstantMobilityNodeContainer
SetReceiveOkCallback on YansWifiPhy
difs-wifi-mac uses companion classes
SetReceiveErrorCallback for YasnWifiPhy
reformatting
separating IPv4 related features to Ipv4NodeContainer class
factorial is a sample of tail recursion
IPv4 related methods were moved to Ipv4NodeContainer class
fixed signed and unsigned mismatches
scheduleSendPacket accepts destination node
some assertions were inserted
learning the usage of ns log module
HelloNsLog.cc and its companion Makefile is an example to use NS_LOG witout waf
Merge branch 'DifsWifiMac'

Conflicts:
	.gitignore