Add Issue.close() and Issue.reopen() functions
Fix several "extra action" function urls

Fixed Group.transfer_project(), SystemHook.test(), and
MergeRequest.post_comment()
Fix Snippet.get_raw() and add Snippet.raw() alias
Fix Team API url
Fix date convert bug caused when a date is None (e.g. snippet expiry)
Tweak README
Update docs
Bump version to 0.4.0