Improve PHP unit tests.

- Add "composer.lock" file to the repository
- Add `autoload-dev` directive to "composer.json" file
- Add instalation of "uopz" PECL extension in Scrutinizer settings
- Add instalation of "uopz" PECL extension in Travis settings
- Convert Exceptions to array on Context::toArray()
- Create methods `overwrite()` and `restor()` on `Arara\Test\TestCase` class to help overwriting and restoring PHP functions
- Move `TestCase` class to `Arara\Test` namespace
- Move PHPUnit unit tests to "tests/unit"
- Remove "bootstrap.php" file used as boottrap in PHPUnit test suite
- Remove "event" property from Context object
- Remove `pcntl_*` and `posix_*` overwritten functions
- Remove checking of `JSON_PRETTY_PRINT` constant from code examples
- Remove PHP 5.3 version from Travis settings file
- Remove support for version 5.3 of PHP
- Update "composer.json" file with the minimum supported PHP version
- Update "README.md" icons (Travis, Scrutinizer, Packagist, Bitdeli)
- Update "README.md" with the minimum supported PHP version
- Use "uopz" PECL extension to overwrite functions

Includes #13