Objj typedef and ivar checks

NEW: @typedef and ivar type warning
Previously, ObjJ was ignoring unknow ivar type. This patch adds some check to ensure the type is either a known class, the current class
itself, a global, a basic JS type or a declared custom type.

In order to declare custom types, this patch introduces the @typedef keyword.

For instance, this will throw a warning:

```objj
@import <Foundation/Foundation.j>

@implementation MyClass: CPObject
{
    NUSuppaType mode;
}
@end
```

This will not:

```objj
@import <Foundation/Foundation.j>

@typedef NUSuppaType

@implementation NUMyClass: CPObject
{
     NUSuppaType mode;
}
@end
```

Declared types are shared accross all application, one type can only be declared once.
Exactly, but if we should have validation here we should have it on all and not only on typedefs. Personally, I don't think we need it.