Adds DROP command for s3_video_stills table

On deactivation of the plugin the `s3_video_stills` tables is not removed.

Steps to reproduce:

1. Install WordPress
2. Activate s3-video plugin and check tables created
3. Deactivate s3-video plugin and check difference in tables

Fix:

Add mysql query to drop s3_video_stills table.
Merge pull request #6 from michaelbragg/drop-s3_video_stills

Adds DROP command for s3_video_stills table