Specify Custom ProxyFactory

My tweet:

> @beberlei Have you heard of overriding the ProxyFactory to allow caching of lazy-loaded entities? Trying to do that now

The majority of our data is quite stagnant and so I was shoehorning the capability of the generated proxies to use a custom class.

My new proxy, in short, will lazy-load the data as normal the first time around, but also stores it in Memcache using an injected adapter. Upon subsequent lazy-loading, memcache is used rather than a call to the DB.

I can't decide if this is better suited for the EntityPersister (which has already been discussed at length), but it seems to fits nicely with a custom proxy.