Add object access support

So here is a script to test:
```php
<?php

require '/usr/local/lib/Hoa/Core/Core.php';

Hoa\Core::enableErrorHandler();
Hoa\Core::enableExceptionHandler();

// Rule.
$rule = 'x[y].foo(2)[0] > f(5)';

// Compile and dump the AST.
echo (new Hoa\Compiler\Visitor\Dump())->visit(
    Hoa\Ruler\Ruler::getCompiler()->parse($rule)
), "\n\n";

// Get the model directly (use the helper, just because I'm tired :-)).
$imodel = Hoa\Ruler\Ruler::interprete($rule);

echo $imodel, "\n\n";

// Context.
class C {

    public function foo ( $h ) {

        return [2.2 * $h, 6 * $h];
    }
}

$context      = new Hoa\Ruler\Context();
$context['x'] = [3 => new C()];
$context['y'] = 3;

// Prepare some functions.
$asserter = new Hoa\Ruler\Visitor\Asserter($context);
$asserter->setOperator('f', function ( $x ) {

    return $x - 1;
});

// First assert.
var_dump($asserter->visit($imodel));

// Here copy the result of the output of $imodel (which is the result of the
// compiler) to compare the result (test the model).
$model = new \Hoa\Ruler\Model();
$model->expression =
    $model->{'>'}(
        $model->variable('x')
            ->index(
                $model->variable('y')
            )
            ->call(
                $model->func(
                    'foo',
                    2
                )
            )
            ->index(
                0
            ),
        $model->func(
            'f',
            5
        )
    );

// Assert again.
var_dump($asserter->visit($model));

// Compare the disassembler of both models (from the rule and from the model).
$disassembly = new Hoa\Ruler\Visitor\Disassembly();
echo $disassembly->visit($imodel), "\n",
     $disassembly->visit($model), "\n";
```
and the result:
```php
>  #expression
>  >  #operation
>  >  >  #variable_access
>  >  >  >  token(identifier, x)
>  >  >  >  #array_access
>  >  >  >  >  token(identifier, y)
>  >  >  >  #object_access
>  >  >  >  >  #function_call
>  >  >  >  >  >  token(identifier, foo)
>  >  >  >  >  >  token(integer, 2)
>  >  >  >  #array_access
>  >  >  >  >  token(integer, 0)
>  >  >  token(identifier, >)
>  >  >  #function_call
>  >  >  >  token(identifier, f)
>  >  >  >  token(integer, 5)


$model = new \Hoa\Ruler\Model();
$model->expression =
    $model->{'>'}(
        $model->variable('x')
            ->index(
                $model->variable('y')
            )
            ->call(
                $model->func(
                    'foo',
                    2
                )
            )
            ->index(
                0
            ),
        $model->func(
            'f',
            5
        )
    );

bool(true)
bool(true)
(x[y].foo(2)[0] > f(5))
(x[y].foo(2)[0] > f(5))
```
That's it :-).

Should close the issue https://github.com/hoaproject/Ruler/issues/8.