Add Web Worker support

Remove all references to window and document. Where the usage of
document is required (IE VB script shims), check that the document
global exists before referencing it.
Add workaround for unknown Content-Length

After removing the HEAD request the actual content-length of the song is
unknown. In the readAtom function, the data.getDownloadedBytes() value
gives a reasonably good estimate for the length of the tag
Force ID4 to return primitives no String objects