Update HalLinks.php

Type in method getid -> getId
Allow HalResource without identifier if identifierName is set to false

This change allows registering a single HalResource without an identifier. See for more details this issue:
https://github.com/phly/PhlyRestfully/issues/109
Typo in $identifierName ($identiferName-> $identifierName)

Fix typos in $identifierName ($identiferName-> $identifierName)
Update getIdentifier function to support $id 0

Changed if clause to explicitly check for $id not being false. Previously when $id is set to int 0 the function results in returning false which is wrong. It should return 0.