Add Chord DHT topology generator

This commit adds a Chord DHT topology generator to the simplemodels
module.
Fix minor typos and add missing documentation
Add test target to main Makefile

This commit adds a test target to the main Makefile that runs test
cases on the core, Java and C++ components sequentially.
Add Mininet example with dumbbell topology

This commit adds an example showing how to create a dumbbell
topology and export it to Mininet
Port C++ Pair fix to ns-3

This commit ports the C++ fix of commit
6234ed41b6ac4a9e68a4a203aab91a853530b0d2 to the ns-3 library.
Fix build issue introduced by newer versions ns-3

This commit fixes an issue arisen in versions after ns-3.16 that
broke the build process.
Update topology file of ns-3 example

This commit updates and adds comments to the topology XML file of
the ns-3 library example.

This commit however does not fix entirely a bug that makes the ns-3
example in versions starting from 3.18 crash due to a failure in
handling IPv4 addresses correctly by ns-3. I attempted to fix the
issue but did not address it completely yet. It will be addressed
by a later commit.
Add Doxygen's mainpage.h to ns-3 library

This commit adds the mainpage.h file to ns-3 library. This
file contains the text that is shown in the homepage of
Doxygen built documentation.
Update ns-3 README file