Two bugs related to inheritance

I've come across two bugs when using Breeze, which both seems to be related to inheritance.

The first one is that entities using inheritance doesn't seem to be properly re-constructed in the cache when loaded using the ```EntityManager.importEntities``` function. Apparently something was missing when remapping navigation properties. The ```unattachedMap.getTuples``` didn't take into account navigation properties which belonged to the base object. This has been fixed with a new prototype function.

The second issue was the ```assignFn``` function would sometimes be null when queying the server with a query extending a navigation property, which used inheritance. A simple check ```(typeof assignFn === "function")``` seems to solve this problem.

//Niels