Add system information to output

Benchmarks are only sound if system information of the test platform is given. This could be added to every output (e.g. as a simple string at the bottom of every diagram).
For a start, I propse the following output:

```
Operating system, CPU, Memory, PHP Version.
```

This string is optional and can be modified or removed with simple commands, e.g.

``` PHP
$phpench->setMachineInfo('');
```
