Allow to pass a Blob to getData (so that only the first 128kb can be processed by EXIF)

For large images it might be more efficient to only process the first 128kb (see e.g. http://code.flickr.net/2012/06/01/parsing-exif-client-side-using-javascript-2/).

To pass only the first 128kb to EXIF one can use `file.slice(0, 131072)`. The result then is of type window.Blob and no longer a window.File, so the check in `EXIF.getImageData` has to be adjusted.

A full example of what this change allows to do:

    var getFilePart = function(file) {
        // From http://code.flickr.net/2012/06/01/parsing-exif-client-side-using-javascript-2/:
        // Using Blob.slice() we pull out the first 128kb of the image to limit load on the worker
        // and speed things up. The Exif specification states that all of the data should exist in
        // the first 64kb, but IPTC sometimes goes beyond that, especially when formatted as XMP.
        var filePart;
        if (file.slice) {
            filePart = file.slice(0, 131072);
        } else if (file.webkitSlice) {
            filePart = file.webkitSlice(0, 131072);
        } else if (file.mozSlice) {
            filePart = file.mozSlice(0, 131072);
        } else {
            filePart = file;
        }
        return filePart;
    }
    
    var filePart = getFilePart(file);
    EXIF.getData(filePart, function() {
        var tags = EXIF.getAllTags(filePart);
        console.log("Got tags ", tags);
    });