remove extra 10px of width added by WP to captions

The extra 10px of width that Wordpress adds to the caption box is not needed and problematic when you are trying to do captions that do not have a "box" layout. Strip out the 10px and folks can add their own styling in when needed.
Merge pull request #24 from kevinlisota/patch-1

remove extra 10px of width added by WP to captions