function createBlockId(partNum) {
        var digits = 5,
            zeros = new Array(digits + 1).join("0"),
            paddedPartNum = (zeros + partNum).slice(-digits);

        return btoa(paddedPartNum);
    }
Implements the Put Block Azure REST API call.  http://msdn.microsoft.com/en-us/library/windowsazure/dd135726.aspx.