rewrite Blob when URL object is not present

Opera 12.10 supports Blob, but [doesn't support URL.createObjectURL](http://www.opera.com/docs/specs/presto2.12/apis/#file). As BlobBuilder.js has its own URL.createObjectURL, it cannot work properly when the blob is not a "FakeBlob" (and URL is undefined).

This fixes https://github.com/eligrey/FileSaver.js/issues/10 for example.