Added additional options for tag background

These options are now available for styling tags:
bgBox - (true|false) background box on/off
bgBoxPadX - (int) x padding on tag
bgBoxPadY - (int) y padding on tag
bgBoxStroke - (string) outline color for bg box
bgBoxColor - (string) color for bg box
bgBoxRadius - (int) radius of bg box corners

For example, this would make the tags look like a bootstrap label (http://getbootstrap.com/components/#labels): 
$('#tag-canvas').tagcanvas({
   'textColour' : '#FFFFFF',
   'bgBox': true,
   'bgBoxPadX' : 6,
   'bgBoxPadY' : 10,
   'bgBoxStroke' : "#5BC0DE",
   'bgBoxColor' : "#5BC0DE",
   'bgBoxRadius' : 5,
});