move parseBackgroundImage to Util; add tests

src/Core.js

@@ -31,6 +31,26 @@ _html2canvas.Util.backgroundImage = function (src) {
     return src;
 };
 
+_html2canvas.Util.parseBackgroundImage = function (value) {
+    var rxBackgroundImage = /([a-z\-]+)\((("[^"]+)|([^)]+))\)/i,
+        match, results = [], n = 0;
+    if(!value) { return results; }
+
+    while( n++ < 100 && !!(match = value.match(rxBackgroundImage)) ) {
+      var def = match[2];
+      if(def.substr( 0, 1 ) === '"') {
+        def = def.substr(1, def.length-2);
+    }
+    results.push({
+        method: match[1],
+        definition: def,
+        value: match[0]
+    });  
+    value = value.replace( match[0], '' );
+    }  
+    return results;
+};
+
 _html2canvas.Util.Bounds = function getBounds (el) {
     var clientRect,
     bounds = {};