Add fixture functionality to system tests

Determine the path to a fixture easily.
Add simple JSON modular service description fixture
Add initial implementation of modular loader
Implement loading of modular service descriptions
Add DefaultGuzzleConfigLoader class
Add PlainTextConfigLoader
Add Symfony's YAML component as a Composer dependency
Add YamlConfigLoader
Use ConfigLoaders to load files in ServiceDescriptionLoader
Add complex system test case
Fix PHP 5.3 compatibility

CallbackFilterIterator is only available since PHP 5.4.
Tell Scrutinizer CI what to analyse
Add unit test for FileFilterIterator
Add unit test for Guzzle config loader
Extract filesystem functionality out of config loaders
Add unit tests for DelegatingConfigLoader
Fix failing tests in CI
Fix missing test coverage
Fix missing property
Fix Scrutinizer issues