Add count accessors to FCModel

This branch adds `+countOfInstances` and `+countOfInstancesWhere:` methods to FCModel. These methods return an NSNumber on success or nil on error. (It wouldn't be difficult to switch to NSUInteger with NSNotFound on error, but I've noticed that FCModel seems to favor boxing values in other places.) As a demonstration, it also adds a call to `+countOfInstances` to the FCModelTest project.