Adding cdn support

[bhaskarmelkani]

This is tested in production environments with and without CDN urls.

[joanniclaborde]

Thanks for contributing! Unfortunately this is not quite complete yet: the unit tests are missing.

Also, I don't think your solution will work with relative paths. See this test for example:

<img src="oops/../images/oops/../ajax-loader.gif" />

It will probably end up like this (notice the missing slash):

<img src="http://subdomain.domain.xxxoops/../images/oops/../ajax-loader.4e26f87c.gif" />