ProxyGenerator eval() proxy code when $autoGenerate is true

ProxyGenerator eval() proxy code instead of writing it to disk when $autoGenerate is true.

Related to [DDC-2210](http://www.doctrine-project.org/jira/browse/DDC-2210)

The idea of eval() the proxy code was suggested by @ocramius in response to the fact that in dev environment, concurrent file writes create errors.

This would also simplify the setup for a dev environment: no more proxy directory to create and make writeable.