added create_adcreative method
added call_to_action field to page post
link parameter of link page post is optional
added some error checking
Merge pull request #28 from GallopLabs/feature/call-to-action-in-link-page-post

added call to action in link page post
Merge pull request #27 from GallopLabs/feature/create-adcreative

added create_adcreative method
Bump up the version after applying pull requests.