Fixed #285.

- Related to following Safari bug which invalidates shorthand css 'background' rule when it encounters '4px'.
- Safari bug : https://bugs.webkit.org/show_bug.cgi?id=37514.
- Solution is to write an all acceptable shorthand css 'background' rule while using 'background-position' for positioning in all browsers and 'background-position-x' for Safari.
Merge pull request #288 from samyakbhuta/bug/285

Tested for FF, Chrome and Safari on Mac.