Added the border-image properties
Added files
Added unit tests for the CSS border-image property / properties
Implemented border-image-outset and border-image-width
Added tests for further border-image subproperties
Implemented border-image-repeat CSS property
Added tests for border-image-repeat
Implemented the border-image-slice property
Tests for the border-image-slice property
Improved background property implementation, started border-image