Allow preprocessing RETS items

I's sometimes necessary to modify the incoming RETS data before it can be mapped.
For example, some vendors return the full name of counties/provinces, while the addressfield stores the abbreviated value.
Because of this, the mapped value is rejected and the listing is stored without a county.

This patch adds a simple alter hook, much like the presave alter hook, but without access to the listing entity so drealty can safetly work with it without worrying about a module having messed with it. If they need to modify the listing entity, they can use the presave hook which is invoked once drealty is done mapping values.