Allow arguments to be passed as references

[raffis]

Currently it is possible to attach arguments to the emitter which will get passed to listeners.
Those arguments get passed by value since internally func_get_args() gets used.

It would be very useful to pass arguments by reference to allow listeners to inject stuff into the emitting context.

This pr makes this possible while still be fully compatible.
A listener may now request an argument as reference:

$emitter->addListener('foo', $value, &$anothervalue) {
   $value = 'bar';
   $anothervalue = 'bar';
});

$value="foo";
$anothervalue="foo"
$emitter->emit('foo', $value, $anothervalue);
var_dump($value); //prints string(3) "foo"
var_dump($anothervalue); //prints string(3) "bar"

[raffis]

Note build fails because of eol php versions on travis, fixed by #87

[frankdejonge]

The next version will not support arbitrary argument passing, instead, it's advised to use event objects with properties on the event classes.