The synchronous option now defaults to false.

It used to default to true even though the documentation specified it
to default to false.
Adding option to change the upload method.

Added 'method' argument to the dialog constructor to influence the HTTP
upload method. It defaults to POST. The default method of the
AbstractUploader class has also be changed to POST.
Parsing status code and status message.

Success and failure handlers now parse the HTTP response code and
message. Furthermore the success handler can now deal with servers only
returning a HTTP response code and message.