Fix Mappable extension

It's need to make it public.

Refactor String extension

To add fileString() method that it will be in charge of return the file as string.

Separate Mappable from ImmutableMappable

To each one of them implement one way of init?(fromFileName file: String).

Separate customer initializers of Mappable's

Separate BaseMappable from ImmutableMappable.

Use BaseMappable instead Mappable

To make it work with Mappable and ImmutableMappable instances.

Add Core implementation

Add ability to create Mappable instances from json files.