Shortcodes: Slideshow: Show the slideshow even when there's only one image to show.
Slideshow widget: Add check for zero images
Shortcodes: Fix caption in one image slideshows.

The jQuery cycle plugin adds `position: relative` when cycling is started. When a single image slideshow is initialized, the cycle isn't started, since there is no need to cycle. Without `position: relative` the caption isn't shown, however. The style is now specified manually.
Merge pull request #1046 from Automattic/slideshow-one-image

Slideshow one image