Suppress php errors and add error event

This PR suppresses php errors via a custom error handler. Currently they are not reported, but a runner event is raised known as error:

```php
$oldHandler = set_error_handler(function($errno, $errstr, $errfile, $errline) {
    $this->emit('error', [$errno, $errstr, $errfile, $errline]);
});
```

Though no steps have been taken to report these now, any custom plugin or reporter could do something meaningful with this event.

Fixes #32 
Do we want any other registered handlers to fire in addition to our own? I feel like we don't. For instance see [this](http://stackoverflow.com/questions/6054068/phpunit-testing-a-custom-error-handling-class).

I don't think we should accommodate testing some globally registered function. I think that if you have something designed to handler errors, test it separately. 

I think we should just remove the reference to `$oldHandler` completely. (see below response)