Revert "Fix the getCurrentSlideNumber to return 0 when it fails to find a slide, fixes #152"

This reverts commit 0930bb94d2ecd3db646e50a88d82d902b2564928.

Conflicts:
	shower.js
Merge pull request #172 from shower/tg/revert

Revert "Fix the getCurrentSlideNumber to return 0 when it fails to find a slide, fixes #152"