Remove ResourceNotFoundException

[mreichelt]

Instead of using ResourceNotFoundException I suggest we should simply return null instead.
Reasons:

• The check against null is pretty common for developers.
• IDEs have built-in support for methods returning null.
• Null is a common use case for jsonapi – common use cases should not throw exceptions.
• This would also solve Add support for finding if a HasOne and HasMany contains data #12 in a jiffy – finding this info is a null check. :)

[kamikat]

When a ResourceNotFoundException thrown from get() it means that the referenced resource is not provided by Document or there is not document related with current Resource object. When there is a null value of a relationship the value of HasOne<>/HasMany<> should be null, and there's no matter with ResourceNotFoundException.

[kamikat]

UPDATE I've update the document. We're now returning null when HasOne/HasMany cannot found resources. ResourceNotFoundException is reserved for Document.find(String, String)

[mreichelt]

@kamikat Thanks!
Document.find(String, String) is a good example for my case: It should behave nearly like a Map in Java – which returns null if a value can not be found.

[mreichelt]

@kamikat I added PR #25 for a simple merge-and-be-happy if you like the change. :)