Rename class SignalHandler to Signal
Rename class Item to Process
Rename "example.php" to "examples/spawn.php"
Improve signal handling

- Use `declare(ticks = 1)` on constructor;
- Moved the default signal handling definition to method
  `setDefaultHandlers()`;
- Create an example of handling signals.
Implement alarm and create an example of
Force to pass IPC to Process' constructor