Remove "event" property from Context object

Since when you trigger an event on some action you pass the event as
parameter there is no need to update Context data specially because it
changes every time.
Move PHPUnit unit tests to "tests/unit"

- Add `autoload-dev` directive to "composer.json" file
- Move `TestCase` class to `Arara\Test` namespace
Use "uopz" PECL extension to overwrite functions

- Add "composer.lock" file to the repository
- Add instalation of "uopz" PECL extension in Scrutinizer settings
- Add instalation of "uopz" PECL extension in Travis settings
- Create methods `overwrite()` and `restor()` on `Arara\Test\TestCase`
  class to help overwriting and restoring PHP functions
- Remove `pcntl_*` and `posix_*` overwritten functions
- Remove "bootstrap.php" file used as boottrap in PHPUnit test suite