Minor improvements  in eventscheduling module

This commit renames EventSchedule method merge_with to add_schedule.
This change improves code clarity.

This commit also implements __add__ and __radd__ methods to the
EventSchedule class so that it is now possible to merge two
EventSchedule objects using the plus operator, e.g. es1 + es2
Minor refactoring all over the project

This commit refactors main Makefile and adjusts the style of a number
of files around the project.
Minor changes to C++ library

This commit adds minor changes to the implementation of the C++
API to fix some minor defects encountered.
Version 0.2.0
Add ns-3 adapter

This commit adds the ns-3 adapter.

At this stage, the ns-3 adapter is capable of correctly parsing both
topology and event schedule XML files and run an ns-3 simulation
based on them.

This commit adds source code, documentation, examples and build
scripts. However, the documentation is not very comprehensive.
It will be improved by a future commit.
Version 0.3.0
Version 0.3.0