started the new per-slide background implementation (#453)
add test directory and simple test document for per-slide backgrounds (#453)
don't transition to first slide background (#453)
support for optional background property overrides (#453)
linear sliding transition option for full page backgrounds (#453)
allow control over background color when using images (#453)
update background test, documented slide backgrounds in readme (#453)
update example deck to use slide backgrounds (#453)
revamp pdf printing to support per-slide backgrounds (closes #453)