update to method addAttachmentToTask

added compatibility fallback to PHP version lower than 5.5.
added mymetype and finalFilename to the attached file.
usage: (ie: $asana->addAttachmentToTask( $taskId, array('file' => $_FILES['datafile']['tmp_name'], 'finalFilename' => $_FILES['datafile']['name'], 'mymeType' => $_FILES['datafile']['type']));
Merge pull request #27 from mnunes01/patch-2

update to method addAttachmentToTask