Full integration of existing ZendDiagnostics checks

**Enables configuration for the followind ZendDiagnostics checks:**

* [DirReadable](https://github.com/zendframework/ZendDiagnostics#dirreadable)
* [ClassExists](https://github.com/zendframework/ZendDiagnostics#classexists)
* [CpuPerformance](https://github.com/zendframework/ZendDiagnostics#cpuperformance)
* [StreamWrapperExists](https://github.com/zendframework/ZendDiagnostics#steamwrapperexists)

**Example config**

```yaml
liip_monitor:
    checks:
        readable_directory: ['/etc']
        class_exists: ['\stdClass', 'Doesn\Not\Exist']
        cpu_performance: 0.2
        stream_wrapper_exists: ['zip', 'zlib']
```

**monitor:health output**

```bash
$ php app/console monitor:health
OK Dir Readable: The path is a a readable directory.
FAIL Class Exists: Class Doesn\Not\Exist does not exist
OK Cpu Performance
FAIL Stream Wrapper Exists: Stream wrapper zlib is not available
```