Add name validation in links

[f3ath]

This is similar to #63. We need to validate link names in setLink() calls.

[martinjoiner]

Have the validation rules been defined? I could help with this.

[f3ath]

The spec is here http://jsonapi.org/format/#document-member-names
Also we have validation logic implemented in https://github.com/json-api-php/json-api/blob/master/src/Document/Resource/ResourceObject.php#L103

[f3ath]

We may consider a light refactoring to make the validation logic reusable across entities. The easiest way I can see is to move it to a trait.

[f3ath]

Fixed in #69