[iOS7] set generated clear colour shadow image on navbar when customizing with backgroundImage on iOS7 to remove the 1 line shadow image automatically. Adds the 1 line shadow into the image if wanted.

Sample customization of navbar in stylesheets for supporting ios7 and previous versions of the sdk :

"UINavigationController" : {
	"navigationBar" : {
            "@ios >= 7" : {
                "backgroundImage" : "navbar-ios7"
            },
            "@ios < 7" : {
                "backgroundImage" : "navbar",
            }
	}
}

"navbar-ios7" is a 64 px height image
"navbar-ios7@2x" is a 128 px height image
"navbar"  is a 44 px height image
"navbar@2x" is a 88 px height image