Add support for base 64 encoded assets and add more tests

This commit also adds tests for the bug fix in dfff2e1f3dd1048be0f185a8e0163535a20b8092.

Also added tests for urls formatted like url('http://example.com/pic.png') and both variations (with and without quotes) for base 64 encoded assets.

Note: Just because it was quicker for me, this PR also includes the commit in #6. Let me know if you want it in a separate PR.