Add support for plain model creation

[hideya]

This PR adds a method to create an LBModel object without setting initial parameters via dictionary.

When to create and initialise an object of a subclasses of LBPersistedModel, the following code would be a typical way. However, it is unnecessarily wordy:

Widget *model = (Widget*)[self.repository modelWithDictionary:nil];
model.name = @"Foo";
model.bars = 123;

This PR is to make the initialisation code simply like the following:

Widget *model = (Widget*)[self.repository model];
model.name = @"Foo";
model.bars = 123;

[bajtos]

@hideya LGTM

[hideya]

@bajtos thanks for the prompt review! I'm going to land this!