ZendDiagnostics file checks integration

Integrated the new file checks from zendframework/ZendDiagnostics#38

=> Also updated the 'full configuration reference' in [README.md](README.md)

**Example configuration:**
```yaml
liip_monitor:
    checks:
        file_ini: ['/usr/local/php5/php.d/50-extension-apcu.ini']
        file_json: ['not_found.json']
        file_xml: ['%kernel.root_dir%/Resources/invalid.xml']
        file_yaml: ['%kernel.root_dir%/config/routing.yml', '%kernel.root_dir%/config/config.yml']
```

**monitor:health output:**
```bash
$ php app/console monitor:health
OK File (INI): All files are available and valid
FAIL File (JSON): File "not_found.json" does not exist or is not readable!
FAIL File (XML): PHP WARNING: simplexml_load_file(): /Users/frne/git/sf2_spielplatz/app/Resources/invalid.xml:3: parser error : Opening and ending tag mismatch: bar line 2 and foo
OK File (YAML): All files are available and valid
```