bundler.js does no Url rebase by default
bundler.js does no Url rebase by default

When bundling CSS files in sub or parent directories, relative URL references will always be broken in the resulting bundle output.

Ex.: 
/Content/Foo/foo.css:
  .foo { background-image:url('images/foo.png') }
/Bar/bar.css:
  .bar { background-image:url('images/bar.png') }

/Content/mybundle.css.bundle:
  Foo/foo.css
  ../Bar/bar.css

Then the resulting bundle output will be:
/Content/mybundle.min.css
  .foo { background-image:url('images/foo.png') }
  .bar { background-image:url('images/bar.png') }

But it in fact should be:
  .foo { background-image:url('Foo/images/foo.png') }
  .bar { background-image:url('../Bar/images/bar.png') }

By leveraging the features of CleanCss, this issue could be easily fixed. 

Some remarks:
- Since CleanCss does not export a 'UrlRebase' module, CSS content might be minified twice when added to allMinCssArr
- The resulting mybundle.css file will still have invalid URL references (only mybundle.min.css will be corrected by this fix)
- Without the 'noAdvanced' option, we noticed some side effects in the processed CSS by CleanCss

Without a URL rebase strategy, we won't be able to make use of bundler :(
ServiceStack/Bundler,ServiceStack/Bundler,ServiceStack/Bundler