Add handler for non-four-sided borders

Hey Colin, 

  Sweettea and Teacup are awesome, we use them on every project.  The need for a non-standard border has arisen several times (i.e. just a top and bottom border, or just a left and right border), so I added a handler to UIView for this.  You can stipulate the side (i.e. :top, :bottom, :left, :right), width, and color of each border using hash syntax. 

- Matt