Support Symfony 2.2 PropertyAccess with BC for 2.1 (closes #218)

By abstracting the property access, we can easily support Symfony 2.1's Form component and 2.2's PropertyAccess component.

The PropertyAccessor service will be injected into transformers if it is available. Tests were modified to do the same, and expect the appropriate exception depending on which implementation is available.
Merge pull request #255 from FriendsOfSymfony/property-access

Support Symfony 2.2 PropertyAccess with BC for 2.1 (closes #218)