Improve benchmark output for various runtimes

On PHP systems and runtimes where SimplePHPEasyPlus is faster than native
PHP, the benchmark outputs misleading results:

```php
$ php -f benchmark/simple.php
For 100000 additions:
    Native PHP "+" operator: 1810.69 ms.
    SimplePHPEasyPlus: 172.39 ms.

=> SimplePHPEasyPlus is 0 slower than Native PHP
```

Instead, when SimplePHPEasyPlus runs faster, tailor the message:

```php
$ php -f benchmark/simple.php
For 100000 additions:
    Native PHP "+" operator: 1833.61 ms.
    SimplePHPEasyPlus: 169.83 ms.

=> SimplePHPEasyPlus is 10.8x faster than Native PHP
```

(I left the "... faster ..." and "... slower ..." strings separate to make it
easier to internationalize this code later.)
Merge pull request #16 from epriestley/benchperf

Improve benchmark output for various runtimes