Set Safari 6 to partial support for bg img options.

I updated Safari 6 to have partial support because it does not support the background sizing offset from edges syntax.

Test case: http://codepen.io/foiseworth/details/BAGjs#pen-details-tab

In Chrome, the logo appears in the bottom right (with a 20px offset from bottom and side) but in Safari it appears in top left and web inspector states the property is invalid.