[PropertyAccess] Extracted PropertyAccess component out of Form

Bug fix: no
Feature addition: no
Backwards compatibility break: yes
Symfony2 tests pass: yes
Fixes the following tickets: -
Todo: -
License of the code: MIT
Documentation PR: -

TODO: adapt DoctrineBundle/PropelBundle to pass the "property_accessor" service to EntityType/ModelType

Usage:

```php
$accessor = PropertyAccess::getPropertyAccessor();

// equivalent to $object->getFoo()->setBar('value')
$accessor->setValue($object, 'foo.bar', 'value');

// equivalent to $object->getFoo()->getBar()
$accessor->getValue($object, 'foo.bar');

// equivalent to $object->getFoo()['bar']
$accessor->getValue($object, 'foo[bar]');

// equivalent to $array['foo']->setBar('value')
$accessor->setValue($array, '[foo].bar', 'value');

// equivalent to $array['foo']['bar']
$accessor->getValue($array, '[foo][bar]');
```

Later on, once we have generation and caching of class-specific accessors, configuration will be something like this (consistent with the Form and Validator component):

```php
$accessor = PropertyAccess::getPropertyAccessorBuilder()
    ->setCacheDirectory(__DIR__ . '/cache')
    ->setCacheLifeTime(86400)
    ->enableMagicGetSet()
    ->enableMagicCall()
    ->getPropertyAccessor();
```

or

```php
$accessor = PropertyAccess::getPropertyAccessorBuilder()
    ->setCache($cache)
    ->getPropertyAccessor();
```

etc.