Bug 1039639 - Parse FLAC metadata

It occurs to me that it would be good to run your tests on files where the metadata is after the first 64mb so blobview has to do an async read.  Or maybe you could just modify core.js to define AudioMetadata.HEADERSIZE that you can set in these tests, and then verify that your tests pass both when you start of with a 64kb chunk and when you start with only 128 bytes or something.