add setter method for duedate

see #162

update isssue' labels

New feature: add or remove issue's label
See #116

attachment service

New Feature: Attachment Service, See - #159

• https://github.com/lesstif/php-jira-rest-client#attachment

add PriorityService

see #155

reporter field is nullable.

See #151

fixed bug - security scheme mapping

• fixed (#148 )
• added json mapping unit test

fixed - security scheme mapping

ref #148

issue resource expansion.

• issue's resource expansion. (ref #145 )

setter for issue security level

• setter for issue security(#143 )

get/delete comment

• added get/delete comment method.(#141)