Better check for undefined value, fixes right arrow showing when it shouldn't

Signed-off-by: James Helly <james@wednesday-london.com>
#207; active slide incorrect after 'remove' operation
support non-video slides with video plugin
Merge branch 'master' of https://github.com/JamesWednesday/cycle2 into JamesWednesday-master
version bump
#230; fix pause when pagers are used
#250; fix tile effect in "rtl" slideshow