Allow ModelMapper instances to be Serializable

This request allows ModelMapper to be serializable.
As an internal requirement in our development we decided to serialize ModelMapper instances in order to speed up redeployments. Some tests are yet needed but before that I prefered to send a pull request and start a discussion.

The following decisions has been taken:
* Method serialization problem is solved wraping it in a Proxy class which controls serialization/deserialization.
* lock Object replaced by a SerializableObject
* computeHashCode method now uses the Class name hashCode instead of Object's hashCode.

Cheers,
Ruben