[Fix json-api-php#3] Implement Attribute Name Validation (json-api-ph…

…p#61)

* Implement attribute name validation

* Add tests for attribute name validation

* Fix type

* Add assertion

* Remove whitespace on blank line

* Remove whitespace on blank lines

Link model fixes (json-api-php#58)

__toString has been removed from the interface

Deprecated ArrayMeta (json-api-php#48)

Improved attribute and relationship name validation (json-api-php#45)

Remove final keyword (json-api-php#42)

Allows developers to extend the class for customised implementations

Static calls check is disabled (json-api-php#36)

NullData -> NullResource (json-api-php#32)

Stabilized the API

Improved Full Linkage check (json-api-php#18)