add form.immediate(false) for "queued" mode

change file input element to accept multiple files (for either mode)

add "queue" event to form (triggered on change to queued files)

add examples/queued.html

Note: queued mode is more like the traditional (non-ajax) UI, with a separate
step for file choosing and triggering the upload. Except, the file upload is
still done via ajax instead of form submit, and you have complete control over
the display of queued files via `form.on('queue', fn)`.

refactor form internals to remove extra queued-files state.
- "queue" and "submit" events simply pass a copy of the array stored in file
  input element. No need to buffer files separately.
version bump