Merge pull request #14 from henriquemoody/tests

Improve PHPUnit unit tests.
Pin versions of required packages
Remove callable checking in favor or type hinting
Use PHPFluent\Callback to run callbacks
Merge pull request #15 from henriquemoody/callable

Use PHPFluent\Callback to run callbacks
Base exception marker interface for the library
Update Process interface

The methods kill(), terminate() and wait() should not throws exception
anymore but return its success or failure.
Update Child class according to the new interface
Update Pool class according to the new interface
Create PHPT functional tests in "tests/functional"