Add simulation class

A Simulation object is responsible for the life of all the elements
intervening in a simulation.
Refactor the stepper class

Refactor the stepper class to use a vector of actors instead of a map
of events (see class graph for detail)
Add actor class

An actor is an element of a simulation which state is affected by the
stepper.
Refactor Event and PeriodicEvent classes

Events are now objects shared over several actors.
Add main method

Method to check the behavior of the stepper (not an actual test). So
far, the behaviors of Actor, Event and PeriodicEvent have not been
checked.
Merge pull request #8 from aMarcireau/event

Event