Support general signals handling in EventLoop.

Support general signals handling in EventLoop.

Now the EventLoop has no support for timers but have general signals
handling support. Signals to be handled can be specified in the constructor
or by calling set_signal() method. Stop handling a particular type of
signals is supported too by calling to the unset_signal() method.

A callback can be specified for each type of signal, or all signal handling
can be done in the handle_signal() method, if overriden by EventLoop
subclasses.