Improve parsing of srcset according to whatwg spec (#74)

* Improve parsing of srcset according to whatwg spec; e.g. srcset="http://example.com/size400,300/img.jpg 640w"

* Trim/normalise the output in order to conform to prior version; solely to keep tests happy

* Add test case for embedded commas in a srcset url as well as support for future possible parenthesis in descriptor string

* Drop 'future proof' test as it causes an error message in test output

test/html/with-relative-res.html

@@ -16,5 +16,6 @@
   <img src="./a.jpg" alt="" srcset="/a.jpg">
   <img src="./a.jpg" alt="" srcset="http://exmple.com/a.jpg ">
   <img src="./a.jpg" alt="" srcset="/a.jpg 3x, /a.jpg 45x , /b.png">
+  <img src="./a.jpg" alt="" srcset="/300,400/a.jpg 300w,b.png">
 </body>
 </html>