Integration of Symfony DependencyInjection Container

This is a first attempt to integrate the Symfony DependencyInjection Container into PDepend.

The following tasks have been achieved:

- Analyzers and Loggers are registered with tags and are fetched dynamically from the service configuration
- The pdepend.xml configuration file is now a Symfony DIC configuration file, allowing to overwrite internal services
- Application services such as Runner, Engine, Configuration and Factories are already served through the DIC
- Documentation for pdepend.xml format updated
- PHPUnit XML Report was removed
- Extensions can be loaded into PDepend using the following `pepend.xml` syntax: https://github.com/pdepend/pdepend/pull/132/files#diff-17b6ad5a75a434a3a081ecce1d045a9cR8 It requires a two-time definition of the `PDepend\DependencyInjection\Extension` class name and also the tag using the value from `getName()` of the extension. I haven't found a better way to do so and allow the extensions to add their own configuration validation syntax.

Todos for @manuelpichler 

- [ ] Fix PHAR generation code to work with new vendor folder