Added Color Attr For Background Color

Added a color attribute to the shortcode that sets the background color in case someone needs something other than the default yellow.
Remove Pre-Defined Background Color
Fixed PHP Error

The div was already in a php string. Not sure why I was trying to echo a variable instead of just putting variable in the string. Fixed it now though.
Merge pull request #2 from fpcorso/issue/1

Added Shortcode Attr For Setting Color Resolves #1