Add support for alternate Hydrator generation methods

This implements two new auto-gen modes for hydrators: AUTOGENERATE_FILE_NOT_EXISTS and AUTOGENERATE_EVAL, per our discussion labelled issue 889.

When I dug into the code, I discovered that \Doctrine\Common\Proxy\AbstractProxyFactory already implemented exactly the same algorithms that I wanted for Hydrators, even including eval() mode.  So I've closely patterned my HydratoryFactory mods after the code in AbstractProxyGenerator. I also put the constant definitions for the different auto-gen modes into the Configuration class, for easy reference.