Don't reject slides with only a background.
Don't reject slides with only a background.

Fixes #180

wanix/showoff,binford2k/showoff,wanix/showoff,puppetlabs/showoff,kublaj/showoff,kublaj/showoff,binford2k/showoff,berkes/showoff,puppetlabs/showoff,wanix/showoff,marrero984/showoff,berkes/showoff,slambert/showoff,puppetlabs/showoff,kublaj/showoff,berkes/showoff,binford2k/showoff,kublaj/showoff,slambert/showoff,slambert/showoff,marrero984/showoff,binford2k/showoff,berkes/showoff,berkes/showoff,slambert/showoff,kublaj/showoff,marrero984/showoff,marrero984/showoff,puppetlabs/showoff