Make resolvePath a little smarter

Use a "relative path" approach:
- if paths have common parts, make sure to not duplicate those parts
- if there are no common parts, just append

Inspired by the functionality of Node's path.relative

src/declaration/path.js

@@ -5,19 +5,47 @@
  */
 
 (function(scope) {
-  
+
   var path = {
     addResolvePathApi: function() {
       var root = this.elementPath();
       // let assetpath attribute modify the resolve path
       var assetPath = this.getAttribute('assetpath') || '';
+      var relPath = this.relPath;
       this.prototype.resolvePath = function(inPath) {
-        return root + assetPath + inPath;
+        var to = inPath;
+        if (assetPath) {
+          // assetPath is always a folder, drop the trailing '/'
+          var from = assetPath.slice(0, -1);
+          to = relPath(from, to);
+        }
+
+        return root + assetPath + to;
       };
     },
     elementPath: function() {
       return this.urlToPath(HTMLImports.getDocumentUrl(this.ownerDocument));
     },
+    relPath: function(from, to) {
+      var fromParts = from.split('/');
+      var toParts = to.split('/');
+
+      // chop to common length
+      var common = false;
+      while(fromParts.length && toParts.length && fromParts[0] === toParts[0]) {
+        fromParts.shift();
+        toParts.shift();
+        common = true;
+      }
+
+      // if there were some commonalities, add '../' for differences
+      if (common) {
+        for (var i = 0; i < fromParts.length; i++) {
+          toParts.unshift('..');
+        }
+      }
+      return toParts.join('/');
+    },
     urlToPath: function(url) {
       if (!url) {
         return '';
@@ -29,7 +57,7 @@
       }
     }
   };
-  
+
   // exports
   scope.api.declaration.path = path;