Support arimethical expressions

See https://github.com/hoaproject/Ruler/issues/11 for more details.

You can test with the following program:
```php
<?php

require '/usr/local/lib/Hoa/Core/Core.php';

Hoa\Core::enableErrorHandler();
Hoa\Core::enableExceptionHandler();

// Rule.
$rule = '-(10 - 2 - 2 - 1) = -y[0]';

// 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.
$context      = new Hoa\Ruler\Context();
$context['y'] = [5];

// Prepare some functions.
$asserter = new Hoa\Ruler\Visitor\Asserter($context);
$asserter->setOperator('f', function ( $x ) {

    return $x;
});

// 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->func(
            '-',
            $model->{'-'}(
                10,
                $model->{'-'}(
                    2,
                    $model->{'-'}(
                        2,
                        1,
                        -1
                    ),
                    -1
                ),
                1
            )
        ),
        $model->func(
            '-',
            $model->variable('y')
                ->index(
                    0
                )
        )
    );

// 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 here is the result:
```php
>  #expression
>  >  #operation
>  >  >  #negative
>  >  >  >  #substraction
>  >  >  >  >  token(number, 10)
>  >  >  >  >  #substraction
>  >  >  >  >  >  token(number, 2)
>  >  >  >  >  >  #substraction
>  >  >  >  >  >  >  token(number, 2)
>  >  >  >  >  >  >  token(number, 1)
>  >  >  token(identifier, =)
>  >  >  #negative
>  >  >  >  #variable_access
>  >  >  >  >  token(identifier, y)
>  >  >  >  >  #array_access
>  >  >  >  >  >  token(number, 0)


$model = new \Hoa\Ruler\Model();
$model->expression =
    $model->{'='}(
        $model->func(
            '-',
            $model->{'-'}(
                10,
                $model->{'-'}(
                    2,
                    $model->{'-'}(
                        2,
                        1,
                        -1
                    ),
                    -1
                ),
                1
            )
        ),
        $model->func(
            '-',
            $model->variable('y')
                ->index(
                    0
                )
        )
    );

bool(true)
bool(true)
(-((10 - (2 - (2 - 1)))) = -(y[0]))
(-((10 - (2 - (2 - 1)))) = -(y[0]))
```
The only BC break introduced is the array declaration syntax (`(1, 2, 3)` to `[1, 2, 3]`). This is the first BC break of Hoa :'-), but since the library is still in the RC state, we can do this.

Also, the `README` is updated consequently.