Remove support for version 5.3 of PHP

- Remove checking of `JSON_PRETTY_PRINT` constant from code examples
- Remove PHP 5.3 version from Travis settings file
- 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
Convert Exceptions to array on Context::toArray()

When converting to array if a value is an exception object, converts its
data to array instead of just create a key for the value.
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
WIP: Add event to Context
Create PHPT functional tests in "tests/functional"