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.
I think acorn takes care of validating identifiers.