1609.4 patch for ns-3 WAVE

This patch is the second of two final patches for the ns-3 GSoC WAVE MAC project by Junling Bu.  The first patch (upon which this patch should be applied) can be found here:

https://codereview.appspot.com/13880043/

This patch contains
documentation, models, three simple examples, a test suite, and helpers.
Besides that, this patch also needs additional modifications to the
wifi module.  The model provides service via a new WaveNetDevice
inheriting from WifiNetDevice, in which the main work is done by several
internal classes.

The four classes are ChannelManager, VsaRepeater, ChannelScheduler and
ChannelCoordinator. ChannelManager stores the WAVE channel information.
VsaRepeater sends VSAs repeatedly and stores VSA tx parameters.
ChannelCoordinator coordinates channel interval operation including
control channel (CCH) intervals, service channel (SCH) intervals, and
Guard intervals.  ChannelScheduler is to assign channel access with four
mechanisms.  Besides that, a new tag ExpireTimeTag is used to support
control lifetime of packets, and a subclass WaveMacQueue inheriting from
WifiMacQueue is used to support AlternatingAccess channel access
mechanism.  An example named MultipleChannelExperiment models the
multiple channel environment.  The test suite tests implemented methods
and features. However, an item for future work is to include more test
cases.