[DCOM-96] Moving proxy generation and autoloading to common

This PR is related to doctrine/common#168. 

In this PR, the `ProxyFactory` has been reduced to an object builder and it's public API has been kept intact (While the proxy `Autoloader` has been moved to doctrine/common). It would be interesting to define what this builder could do with the `ProxyFactory` to get its own customizations introduced.

Also fixes [DDC-1734](http://www.doctrine-project.org/jira/browse/DDC-1734)